how do I tell if my hamster is sick?

Answer 1

Feed him to your cat...if the cat gets sick then the hamster was probably sick.

Answer 2

I remember when my hamster was sick, he just stopped being active, even at night. Little activity = there might be something wrong with your hamster.
Please - go see a vet if you need to. Hamsters are so fragile and mine died because I did not take action quickly enough.