In reality they can't, these anti doping organisations have limited funds and there's only so much they can do. It's a constant arms race, people find new compounds or new ways of masking existing compounds and then these anti doping organisations have to find ways of testing for it, it's a race where they are always lagging behind. But they've made strides in recent years with the introduction of biological passports and the practice of now storing samples for years to test years later with new methods of detection which is catching a lot of cheaters. The most recent was a Russian 3K runner who won gold at the European Indoor championships 9 years ago. But even that is a limited deterrent as it's retroactive the athletes that have cheated have still financially gained from cheating up until they are exposed and those clean athletes that lost out on medals have still lost funding or sponsors because they failed to medal due to these cheaters. As for boxing itself, it's even harder due to the fractured nature of the sport. You get banned in one country, well you then fight in another country. Base yourself in the US and you avoid year round testing, the total lack of a single boxing authority makes it impossible to create and properly regulated anti doping program that is consistent globally. Honestly I suspect boxing has a doping problem as bad as sports like Cycling and Rugby which are notorious for the number of doping cases even at the amateur level.
With every advance in technology to catch steroid users there' always an opposite and equal advance in how to cheat past it. Fighters know how long they can take ped's and how long to be off them before fight testing. Day of fight weigh ins might help somewhat.
If you see any fighter past 35 that looks fast and with lot of muscle, he is on PED. Probably entire top 10 is on some kid of drugs. I mean Fury was proven to have drugs and still holds belts, and Joe Rogan actually said Naganou was born that way. Yeah, where were all those killer in 1950s?
You can obviously make it much better, because at this point it's pretty low quality in boxing (always has been), because there's no oversight and nobody wants to pay the bill. But it's even more about the penalties are laughably low or not even there. The idea that Miller could be caught doped up on the entire pharmacy twice after being kicked out of kickboxing for it, and still be allowed to box is a travesty. Fury getting caught on nandrolone and nobody knowing about it until months after he won the title, being allowed to keep it and the win in the fight he was busted in. Floyd getting busted red handed masking for the biggest fight of the era and completely getting away with it. If there's zero punishment for getting busted, then what's the point?
The testing is not good enough. Most boxers will gladly take knowing everyone is taking. JMM moved up and became the WW version of Earnie Shavers. Literally knocked Alvarado across the entire ring with back acne. If he wasn’t caught, I doubt many will be
Some are no doubt corrupt, currently Spain are in trouble for what could be major infractions in regards to their anti doping program. Which explains where all these suddenly good Spanish track and field athletes have emerged from nowhere when the country doesn't have a history of doing well in this sport. They are definitely turning a blind eye and allowing athletes to compete despite an adverse test and have been doing so for the last 5 years apparently.
Ultimately in Boxing the budget for PED testing is risible, so as long as fighters are advised properly they are free to dope as they please. If boxing wishes to improve more frequent and advanced tests are needed, as are intensive follow up tests to an "abnormal" result, as well as a biological passport similar to cycling etc.
Catching them is a challenge, but they do catch some, so make it a lifetime ban for those caught using PEDs, no appeals, no dispensations, make a real deterrent.
That won't ever happen because a life time ban in the courts would require an even greater burden of proof that it was done and done knowingly and the 2nd part would see every doping case fall apart as well as bankrupt many anti doping programs with legal fees for every case that's dragged through the courts. Right now most found cheating will challenge to a point to feign innocence, but eventually accept their ban knowing they'll be back in a couple of years. If a life time ban was the consequence every case would be dragged all the way through the courts.
Challenge away, if the rule is zero PED in fighters system, and a PED is found then they are guilty as charged, being allowed to box professionally is not a god given right. This is not a mater of someone juiced being able to beat competitors over a 100 metre race, this is a juiced up individual possibly killing a non juiced guy in a boxing rimg.. The lifetime ban idea is stated by many ex world champions and notables in our sport.