Proof and the legitimacy of what is said.

A schizophrenic with "demons" stalking them is very different to a child with imaginary friends or someone who is telepathic.

A schizophrenic will have thoughts and beliefs that don't coincide with reality, see things that no one else (including people who actually do see spirits) can, and are often unable to think straight and are unable to prove that whatever delusions or hallucinations they have to be true and real in any shape or form.

Mental illness is a very obvious thing to people who know what to look for.

A child with imaginary friends will often keep their "friends" associations with them as someone equal to themselves, another 5 year old who likes to play on the playground, or have tea parties perhaps. It is all in the imagination so, unless the imaginary friend is 30 years old and is teaching the child how to read and write or telling the child to tell *name of relative* that the person misses them and that they are sad that they died so soon, it is most likely an imaginary friend.

Telepathy is a tricky one, because it can be easily confused with your own thoughts, an auditory hallucination or simple coincidence.

With telepathy, I would say that testing yourself and other people by "guessing" what they are thinking on a regular basis will hone your skill, or make it clear that it's just one of the things I stated above, rather than telepathy.

This is all just my opinion on things. Other people will say differently.