How can you tell if your hamster is in pain?

When you hamster is hurt or sick, it usually has something unusual going on about it and acts lethargic (unactive). If it's hurt it could also act weird. If it shows signs of being sick, look for symptoms and look up in a book or the internet to see what could be wrong.

Here's a website with a list of hamster sicknesses:

http://www.hamsters.co.uk/hamster_health.htm

Hope it was helpful!