My character was bitten by another human; how would a doctor find this out?

Yeah. Human teeth are pretty specific looking and the marks they make are very recognisable.

Yep, what the rainbow-pooping unicorn said. They are so distinguishable that doctors can tell human bite marks even when they haven't broken the skin, just left a mark.

I know this because there was a weird local child-abuse case several months back where one of the parents had bitten a child numerous times all over. So sad

It is pretty easy to tell a human bite from an animal bite. Totally different shape.

Human bites are easily detected/identified, and residual DNA is almost always recovered. Thus, confirmation of a human assailant is fairly conclusive.

Speaking as a mum who used to help out in a playgroup, the marks made by human teeth are really very distinctive even when all you have are a few dents/bruises and you are not a medical expert.

Just in case anyone is tempted (as I was) to repeat the urban legend that the most common source of rabies in humans is the bite of another human, check your sources. Bats are the most common wild animal to give people rabies, and cats are the most common domestic animal. Off topic I know but interesting.

In many cases, if the bit mark is clean, its just like a set a fingerprints for when they find the culprit...