it's really hard to explain how you know what kind of kin you are, or if you actually are otherkin.
let's start with the otherkin thing. if you are one, you typically feel like you don't belong into a human body and into human society. you see yourself and your surroundings like from a 3rd persion view, there are human traits you don't really understand and you feel you want to just run away without being able to say why and from what. in some situations you might want to growl, hiss, bite, fly away or get invisible... so there are reactions that you cannot explain. you just have that odd feeling of not being like all the others. many otherkin also feel limbs that aren't there like wings or claws.
the more difficult part is to determine what kind of kin you are. i have heard statements like "i have black wings, i must be a demon", which is totally wrong. you have to search deep within yourself. what are the differences between you and a "normal" human? do you feel strongly connected to an animal? are you empathic and is it your wish to guide and help others? are you drawn to chaos, do you despise everything that talks of itself as "good" and "righteous"? do you love nature, walking in forests etc.? these are just sample questions that should help you to get an idea of your kin type. it also helps to read a forum like this - do you feel "at home" in a certain section of the forum?
unfortunately nobody can tell you whether you are kin or what type of kin. you have to find that out for yourself. and it may take weeks, months, even years, so be patient. people who tie themselves down to a certain type too fast may eventually realize they have been wrong all the time and have a hard time coping with the results.
i have always known i am a demon. talking to other demons in various forums has given me more info on our kind, which, however, didn't seem to be new to me, just like old memories coming back again. i know what it is like to float around, without a body, how people react when (if) they see me etc., it feels strange to eat, drink and especially to sleep. but i also know the advantages of a human body - communicating with humans is easier, they aren't afraid of you and suddenly you understand concepts that were unimaginable before - pain, illness, physical love... the mere fact that i am so fascinated about those things excludes most kin types
if you are a potential otherkin searching for answers, i hope i could help you.