It's hard to say. The athletic commisions of each state, and/or the sanctioning bodies are supposed to do pre-fight and post-fight drug screens. This doesn't always happen, and alot of times you don't even hear of such a thing until there is controversy, or unless Don King is involved.

Alot of bigger names have been busted{Toney,Vargas,etc.}, so that gives the impression that tests are being conducted. But, money can make people forget about such tests. Sometimes they do both tests-Tyson vs Golotta, Mike was clean before the fight, after the fight he tested positive for marijuana. Sometimes they only do the after test, Toney wasn't tested before, but afterwards tested positve for steriods.