Is someone who takes an as yet undiscovered by WADA performance enhancing "supplement" clean?
The likelyhood of that is pretty minimal. Performance enhancing drugs cannot be discovered by some guy in a shed. Tends to require significant resources, a decent lab or 7, and some good research personelle. Way outside the budget and scope of most pro tour teams. Most of the agents are known to science, if you can't test direct, you can detect masking agents.
Plus there are only so many biological 'parameters' you can improve - which are tested.
The tricky one is blood autotransfusion. Harder to spot. You kinda need plod to find a fridge/car boot full of blood bags there really. It def is not systematic anymore.
Doping in training, or out of season, is possible. Risky, but possible, provided the half lives of the compounds you are using are known with some degree of confidence, plus of course, you know when you're likely to be tested next. Plus you're open to refinements in testing - a la Contador.
Cadel - clean as an autoclaved kazoo.