how do you know when somebody is interested?

Dear Rent a pal,

I have a friend who asks me if her new male friend is interested in anything more from her. She really likes him but sometimes he is all over her and then at the drop of a hat he seems vague and a bit cold. I am useless at trying to work men out so thought i'd ask you the question!! Please help me help my friend
andy

Dear Andy
If your friend's friend is sometimes "all over her," then it's clear he is interested in sex, at a minimum. If he becomes vague and cold at the drop of a hat while they are being physical, then he may be psychologically unstable. If he becomes vague and cold only at other times, then he's probably just not interested in anything beyond sex.
Your pal
Paul