I have taught a class a few times which focuses on helping people learn to differentiate between several types of spirits they may be interacting with. The point of doing this is that what a ghost can or can't do is different from what a fairy can or can't do which is different again from what a third type of spirit might be capable of.
This is by no means a foolproof process but was intended to help people just starting out in spirit work. This guideline is based on my own personal experience and study, and so please keep in mind that this will by necessity therefore be based on spirits that I have myself dealt with. I wouldn't be confident that this will hold up without testing against spirits entirely foreign to me, although theoretically it should hold true in a general sense. My point being that there is a bit of caveat emptor with all of this.
There are many features of spirits which are shared by various groups, such as the ability to be seen or unseen, the ability to physically touch a living person, appearing in various forms, making people feel a sense of being watched, or communicating in dreams. There are also some qualities which are unique to different types and that's what we will focus on in the following sections.
Ghosts in this context are the disembodied spirits of a dead human which is still lingering on the earth-plane. There are other options for human spirits, ghosts are only one, but it is important to be able to recognize the presence of a ghost when there's one around. In my experience a ghost is fairly limited in what they can do to influence living humans and the harm they can do; they're greatest power is startling people and the person's own fearful reaction. That said it is possible for a ghost to possess a medium who is open and unshielded and people can be scratched, tripped, have their hair pulled, and things along those lines.
1 Ghosts are particularly noted for the way that they will influence the temperature of a room by making it drop, the theory being that they are somehow transmuting the heat into energy they can use to manifest or otherwise interact with our world.
2 They are also known for affecting electronic equipment by turning things on and off, communicating through radios, and draining batteries*. If electronics are being interfered with or messed with ghosts are a prime candidate.
3 Dogs are sensitive to ghosts - some so-called 'ghosts hunters' even use dogs in their investigations because of this - and will react when ghosts are around. This reaction usually includes barking at odd spots that seem empty, refusing to enter a room, or seeming uncomfortable in an area.
4 Things moving, usually involving a slight touch, such as something falling after being tipped off a shelf, sliding across a smooth surface, being knocked over or shifted out of place. What distinguishes a ghost doing this from other spirits is that ghosts have a limited range when it comes to interacting with the earth plane so its usually either small moves or things that involve a tiny shift or pressure to gain the result.
5 Smells that can indicate a ghost is around may include scents that were hallmarks of the person when they were alive. For example if a person wore a particular perfume every day when they were alive after death living humans may smell that scent when the ghost is around.
6 Ghosts can manifest visibly, but rarely fully to those without an ability to see them. They may be seen when they make the effort as amorphous shapes, as partial figures, or as mistlike forms.
Identifying Non-Human Spirits
I use the term non-human spirit as a bit of a catch-all for spirits that aren't human ghosts and also aren't fairies. These may include beings normally referred to as demons, shadow people**, poltergeists, or negative entities. My experiences here tend to focus specifically on dangerous or harmful spirits, so keep that in mind. There may very well be other types of non-human (non-fairy) spirits that fall outside the range of what I'm talking about below. Also its worth noting that just because these spirits can be and may be dangerous doesn't mean they exclusively are; it is possible to have positive interactions and relationships with them and some people do actively work with them. That falls outside my purview however.
1 Spirits can and will lie but there's an old belief that if you ask the same question three times the spirit must answer truthfully the third time. There are other means of obtaining honest answers as well, including asking a question you - and theoretically only you - would know the answer to, or asking the spirit to speak truthfully in the name of a higher power the spirit would obey.
2. Animals will tend to react badly to negative entities being around. This can include growling or barking aggressively (dogs), hissing (cats), or equivalent reactions in other animals.
3 Bad smells tend to be associated with these types of spirits particularly sulfur/rotten eggs or garbage.
4 Silhouettes, shadows, and dark shapes are all appearances that these types of non-human spirits can have. I have also heard of them mimicking the appearances of familiar humans or intentionally taking on forms that frighten the viewer.
5 These types of spirits are more prone to physical interactions and violence than other types, particularly scratching; I have heard claims the scratches will always be in a certain number but I don't find that to be true. They will however usually bleed and usually appear inexplicably sometimes while the person is looking at the spot.
6 These types of beings are less prone to audio phenomena than other types but things like growling may be noted. Usually the growling is sourceless, occurs near the person, has a threatening feel, but isn't followed by any action.
7 These spirits also can and will physically throw items, even larger objects.
Identifying the Good Neighbours
The Good Folk are around humans more often than humans may believe. I think that especially in the US it's not uncommon for interactions with fairies to be mistaken for other types of spirits particularly non-human spirits. I know I've seen more than one episode of a paranormal show where the people thought they were dealing with a ghost or 'demon' and it seemed clear to me it was the Good Neighbours.
1 Items disappearing entirely. While other types of spirits will move items - and fairies can as well if they choose to - fairies are particularly known for taking items entirely.
2 Cats tend to be sensitive to the presence of fairies and will start acting strangely, even for cats, when they are around.
3 Sourceless music, the sound of horses and hounds, and conversations are also hallmarks of fairy presence. Although ghosts can sometimes also manifest with sound phenomena its usually localized and connected to a specific place or event, such as the sound of guns on a battlefield or piano music in a concert hall. With fairies the sounds will be entirely out of place and may make the listener feel a strong emotion.
4 If a fairy chooses to be seen you will see them but they can also sometimes be perceived as motion in the periphery.
5 Fairies can't lie but they are masters of semantics. However this verbal honesty is a notable difference between fairies, ghosts, and other types of spirits and can be helpful to remember if you are engaged in any kind of communication with them.
6 Fairies are or can be more physical in our human reality than human ghosts and interactions with fairies can potentially be very tangible. Don't underestimate that possibility.
7 Food spoiling is another thing to make note of as a sign of fairy presence. If they are around and if they feel they are not being given what they are due or you are speaking badly of your own possessions/food they can and will take the substance from it so it spoils. I don't know any other spirit that does this.
If you are engaged in active spiritual work that involves working with or dealing with spirits these basic guidelines should be helpful in sorting out what you may be dealing with. All of these categories are fluid though and these are only meant to be guidelines not hard rules. The more you do this the more you get a sense of certain things where it becomes almost reflexive.
If I were meeting a spirit for the first time, or if someone where describing a spirit they were dealing with, I would look at these rough qualities to consider more or less what I thought the spirit was. You can of course always ask it the requisite three times what it is. If its a ghost then I engage with it as a human without a body. If its a non-human spirit then I usually look at trying to establish its trustworthiness and whether I want to deal with it. If its a fairy I'm more respectful to start and would look at whether it seems interested in me, whether that seems healthy for me or not, whether that's a friendship/ally I want to pursue, etc.,
From there if it is something I'd like a connect with then communication is in order. Otherwise I either look at leaving its area, seeing if we can get along by ignoring each other, or getting it to leave mine if its a newcomer there.
Whether its a ghost, non-human spirit, or fairy there are certain things that are universal. Iron is mentioned across folklore as being a ward against all types of spirits as well as baneful magics, with salt a close second. We're not talking about vampires here but having firm boundaries and not inviting things in that you don't want in is also a good practice, and if you banish something you have to be resolute about wanting it out. When you first encounter any spirit its a good idea to reserve judgment if possible until you can sort out what you are dealing with. If that's not possible then initially erring on the side of friendly caution is a good idea while you feel things out. Not everything is dangerous or out to hurt you but certainly not everything means you well either; act the same way you would with a new living human.
*I'd just note that people have also observed batteries draining at locations associated with fairies. However generally speaking the phenomena is more widely connected to ghosts.
**I personally believe that shadow people are more a category than a specific type of being and that there can be some fairies or ghosts that are identified as shadow people. However for our purposes here I am slotting them in with non-human spirits.