- You go out of your way to see or hang out with that person.
- You are starting to worry more about your appearance.
- When writing a list of positives and negatives about that person, the positives win...by a landslide.
- You find yourself being clumsy or awkward around them.
- You find yourself getting tunnel vision (you can't focus on anything but this person).
- They are not around and you find yourself smiling.
- You start asking his friends about him.
- Your friends are getting sick of you talking about her.
- For some reason, you can not stop teasing this person.
- You hate going to your Physics class but lately you don't seem to mind.
Crush: an intense but usually short-lived infatuation (http://www.dictionary.com/)
Ahhh! The crush. Books have been written on it. Songs have been sung about it.