how can you tell if someone is interested in you? at what point does a relationship between two people cross over the line from friendship to attraction? when is it no longer discussion, but flirtation? could someone please tell me?

is it a confession from someone that they find your looks appealing? is it the accidental rubbing of that someones hand against your leg? or is it when that someone tells you he's been dreaming about you? is it the obvious pleasure of that someone when you rest your arm and head upon them after a day of hard and stressful work? is it the simple fact that when that someone talks to you, they look right into your eyes, and don't even notice the other people around you? or that when that someone does look into your eyes in that manner, your face lights up, which makes them smile?

if it is any of these things, i am in quite a pickle as to whether i should make the first move, or whether i should wait for him to.