If you are passing them by watch their eyes if they look at you then:
Look down: They like the way you look
Look immediatly to the side : They have yet to decide
Look Up as if rolling there eyes : sorry no love today
This of course is not full proof. Also the rules do not apply if you have had any significant conversation.
Other than this and the normal signals (Ex. the touching, long looks, the excess smiling and extra long conversation) I no of no other way to tell. However I do know a couple of ways to make yourself more attractive and almost make someone fall in love with you.