Passing the tests is easy, you stop taking the drugs in advance of the tests so that they clear your system. That's how Ben Johnson, Linford Christie and so many other athletes passed the tests for years. Out of contest testing is fairly easy to avoid as well, although Mo had to miss two tests in the year before London 2012, one away from a 4 year ban.
The circumstantial evidence is overwhelming, I'm afraid PRSB.