I don't think it's an exact science. I think "bioload" basically means how much a fish eats and how much waste it produces in relation to the size of the container it is in. So, two goldfish in a 50,000 gallon pond would be a small bioload compared to, say, a school of 10 neons in a 2.5 gallon nano tank.
Although, as a general rule, the fish's size is probably a good indicator. The bigger the fish, the higher the bioload, assuming that the tank size is in an adequate range for that type of fish. At least, that's how I understand it--someone who knows more can correct me if I'm wrong.