Lets assume he does have a bite, what you want to look for is signs of infection. Swelling, redness, puss. Anything that looks out of the ordinary. If he looks fine after a couple days then he probably is. Bites happen from time to time, but these snakes are very tough, they can handle a small bite here and there. Just keep the cage nice and clean and the risk of infection should be next to nothing.
I've only had one rat bite my snake. It was right on the top of her head. I left it alone and it healed wonderfully. You can't even tell she was bit! And it was a pretty nasty bite.