Personally I think they would be few and far between. One only has to look at cycling to realise how easily it is to get away with doping, and they're by far the most tested athletes in the world. Albeit they're probably micro-dosing and only getting marginal benefits from doing so.
Whereas UFC fighters/NFL players/soccer stars don't get tested even 1/10 as often, so it would be difficult to believe that serious doping isn't very commonly practised, right?
Surely one would have to be very naive to think otherwise in my opinion