Just like "All roads lead to Rome", all of your thoughts lead to him/her and you start accepting them for who they are, that's when you guess "gosh, am I falling in love with him?" And chances are that when you ask yourself that question, you are already in love.
It is hard to explain but you definitely know that it is way different from having a crush on someone. Also infatuation usually wares off fairly fast. Infatuation gets you to talk with the person or you develop a crush on them.