Crush. Everyone has one (or many). It is normal between you and me. Mainstream crush is having crush on celebrities. While exclusive crush is liking someone whom you place a (slight) hope of getting her/him.
Well, how do you know you're having a crush ? It comes unnoticed, at the beginning. You simply like the person, either in real or virtual world. You could like him/her for the good look he/she has or you could crush on that person's writing, the way he/she talks, his/her wealth, or the way he/she laughs. Yes, sometimes, people crush on people for weird or absurd reasons like his/her signature, or his/her 'hot' body etcetera-etcetera.
However, the expression of excitement is different between men and women. Usually, men do not (really) show or make it evident when they have a crush on any woman. They probably just share with some close male friends. For some, they just have the balls to approach their crush. That's what men (usually) do. Women treat their crush feeling differently. Generally, they don't (simply) approach. Most women reserve that feeling to themselves, or tell to some close female friends. Probably, they like it better as nice little secret for having a crush on any man. However, for whatever reasons you crush on someone, remember, crush is feeling and feeling is fragile. Handle it with care.
We are not talking about falling in love here first. Having a crush is the first step to seriously fall in love. To crush is to gamble with your feelings. If you're seriously crushing on someone, the failure will bring serious hurt, too. (Only if you place hope). Some of us, they just have that feeling to almost anyone. Well, that maybe is just liking somebody. There's a fine line between simply liking somebody and crushing on someone.
All in all, you just know when you crush on somebody.
[ My point of view is not 100% for you to agree. There is a quarter for disagree. There's always your point of view.]