Look under his/her tail. If he is a "whole" (unneutered) tom, it should be obvious. You'll see his testicles. If he has been neutered, look closely and you may see two very small remnants of the sac. Otherwise, it's probably a female. Don't feel bad, though, if you're wrong-- even the experts are fooled at times.
Thanks for taking this kitty to the vet. Sounds like you're thinking of keeping him--err---her.