Can't say anything from personal experience but I remember reading somewhere that based on a psychological study, a crush only lasts for a maximum of 4 months. If it lasts longer, it's probably love. Now I don't the validity of that 'study' if even there was one.
I think you would just know. We have crushes all the time.. on actors, sometimes even classmates and all. But when you actually fall in love, the feeling would be a lot stronger.. and sort of all-consuming... I don't know if that even makes sense... I think when you find "the one" you just know.. There are a lot of people you can live with, but there's only you can't live without.