Have the following conversation..
you : We're pretty good friends aren't we?
him : Yes, I suppose we are.
you : Do you ever wonder if it might be more than that one day?
..if at this point he laughs and say 'naaaa, course not you silly sausage' and ruffles your hair as one might do to a cheeky scamp, that's not a good sign.
If however, he looks a bit embarrassed and says something like '...err I dunno - I never really thought about it...don't see why it couldn't'
That's a definite good sign.
At any rate, it moves the issue forward in a positive but not too forceful or desperate looking way.