How do you know who your friends are?

To answer the question directly, I don't. I really am unsure who my friends truly are.
For two or more people to be considered friends, there is no criteria, there is no way of evaluating if they truly are friends, so how am I suppose to know who is truly my friend?
Although there are some people who I am on better terms with and get along with rather well, I have no way of telling if they feel the same way about me. People have different personalities and humans never always tell the truth. How am I to know how others feel about me? I cannot tell as they might not tell me anything; even if they do, how am I to know if they were telling the truth or lying in my face?
Friendship is really complicated a thing, sometimes it takes minutes for two people to get acquainted; those friendhips sometimes last, at other times it simply dies out after a while. It is impossible to foresee how long a friendship can last, similarly to how it is impossible to see if the friendship is true.
Friendship is intangible. Intangible things have no way of being confirmed. That is what I believe. However, if you believe that intangible thing is real. If you believe enough and time has shown it true, it may just be real.
Time is the only way to tell if someone is truly a friend, although it may not reveal everything, it reveals things that are important for you to believe that one is truly a friend. Although it may not be truly true, its true to oneself, and I believe that may just be good enough.