You can never be sure until you ask him. Men love to flrit and play you around it makes them feel good about themselves not that i'm bitter or anything.
Ask him during converstaion what his 'type' of girl is....then if it matches you then you are halfway there....being a guy I always seem to get told (insert random girl's name here) used to fancy you and I'm like, yeah you tell me now, six months later.....
Drop the odd hint. Such as,'I was going to the ...... with a mate of mine and she's cried off'. Leave it at that and see what happens. If he's interested he will soon find a reason to explore further.