Interesting question. I'd assume Lance Armstrong was taking steroids as a replacement for testcancer. So don't know? It will become an issue when golf starts testing, as there are a lot of older golfers playing that could likely get HRT.
You CAN test the difference between most injected test and natural. It involves the type of carbon used in injected vs man made. I believe they do it at the olympics. No doubt some high tech labs are making it with the different carbon to beat the test.
My guess is every competitive athlete is using slin. It would be very difficult do test for, I would think. There are minor changes in injectable kinds, to increase the active time. So it might be possible to test for them. But the 1/2 life is so short, don't know if it's practical.