The reporting on this issue tends to be bad as writers prefer the sexy angle of a bunch of people working outside the law that will never be caught. Arnold was talking openly about these things on Usenet forums. He wasn't exactly hiding.

This article does a decent job of explaining things.

Arnold was big into finding precursors that the body would turn into the steroids because precursors existed in a gray area of the law at the time.

Before this baseball season four players were caught using stanozolol which was surprising because stanozolol is a well-known steroid that a technician would know very well. So either they're dumb and got suckered by a dealer or what I think happened is that they thought they changed the steroid enough but the testing changed.

Peddlers rely on the limited time these technicians have to analyze samples. The mass spectrometer is one of the most important tools and the peddlers know this. So a technician will take the compound, break it up and weigh the molecules. He's looking for a specific weight for the known steroids so if the peddlers take a chemical chain and replace a methyl with an ethyl the weight will be different and the technician won't recognize it as a known steroid.

But WADA accredited labs are tasked with changing their tests and they have introduced advanced mass spectrometers and biomass tests but those take additional time to examine a sample.

When Anderson Silva was caught this year, the media and fans freaked out because his positive sample was taken several weeks before the fight but the results didn't come in until after the fight. They were all outraged that the results didn't come in sooner which is what the peddlers want. They want people to demand quick test results instead of accurate results. They don't want the labs using the in-depth tests and the public really needs to get their shit together on this one and stop thinking fast is better.