It's hard to know the percentage because these 'brave warrior' boxers are too scared to blow the whistle and expose it all, even when they retire. I would guess it's probably like with Bisping in MMA, some clean fighters surrounded by roiding cheaters. Probably the clean fighters are a minority, but who really knows. Also there are levels of PED, and I assume many don't do it all the time, just for certain fights. So at times they are 'clean'. One problem is that people really do believe the 'clean' pass from drug testing is legit, when it obviously fails to catch so many. The best way is to just use common sense and when something looks clearly unbelievable it probably isn't natural. If fighters are bulking up unnaturally(AJ, Wilder, Joyce, Canelo etc.) or their abilities are magically increasing (Canelo endurance and power) or they are defying weight classes ridiculously (Pac, Canelo) or defying age ridiculously (Mayweather, Pac v Thurman), even if they just have some ability they seems way too incredible compared to everyone else (Wilder power, Pac power for his size), you probably have a PED cheater, regardless of what the testing shows.
I suspect boxing, especially in countries like the US which don't do random resting 24/7, 365 days a year, is one of the worst sports for PED use. Probably right up there with sports like Cycling. It's so easy to beat the system as you can just cycle off the PED's between fights when you are not tested. I'm actually shocked when boxers do get caught, given how doping itself has become a science, which probably just goes to show how boxing is still not up to date when you comes to some modern training methods and tied down by tradition. But in general I think the idea that everyone is using PED's is as likely as nobody using PED's. There are athletic records from the 80's that still stand today because the testing protocols back then were dreadful and it's just accepted that athletes then were cheating. If everybody was doping now in athletics those records would have been toppled years ago but the clean athletes now still can't beat those records despite training methods becoming superior and technology becoming better in terms for running shoes, lighter weight clothing etc. I'm sure there are clean boxers even at the top level but then again I'm sure some are cheating. It comes down to human nature, some people just would never cheat, it wouldn't even cross their minds to do it and if offered the chance to use them would refuse immediately without even considering it as an option. But then there's other people who will always look for a short cut and try to gain the system to their favour as that's in their nature.
This is anecdotal as my lecturer would say....but I reckon there will be some clean fighters in the sport. Undoubtedly there will be many performance enhancers though. Hate to think what the percentage split is in reality.
Unless under regular testing its easy to evade detection. Thats why it's rife in other sports like baseball and nfl and Putler's Olympic squad.
Performanc Enhancing Drugs. All the 7 figure guys. Legal or not so legal ones. Everybody is on da roids. I don't worry about that particular cheat.
PEDs aren't cheap "Nutritionists" are not cheap either. Neither and strength and conditioning coaches. The younger guys on the way up are probably the cleanest simply because they don't have access to the good stuff or the smart people who can tell them how to cycle on and off a course of PEDs smartly.
Steroids aren't going to make up the difference They can help...but not overly especially when testing is in place to stop fighters turning into radioactive monsters.