I don't know anyone who hasn't had a crush on someone or has been crushing on someone so here are some ways to know
If you are a guy/girl here are the crush symptoms
1. plays in hair
2. pupils goes pig when they see you
3. nice to you (duh)
4. touchy ;)
5. body language
6. interest in things you like to do
8. treatment (how he/she treats you other than other people)
That's just a few if you have any questions please ask bellow