I was thinking about this some more and perhaps it is lacking in logic, but the most believable source of evidence is precedent.
Anywhere you look, the discussion invariably comes back to: has this happened / been done before / by anyone else?
I think this partly stems from the argument that seems to follow the logic of, "Sky are doing this, therefore it is evidence that they are clean".
Painful.
When someone says, "He has never tested positive" - this is indeed evidence. However the precedent for using this as a reliable piece of evidence has been irrevocably destroyed by people like Kohl and Lance Armstrong.
When you see an unimaginable leap from pack fodder to fourth in a heavily doper filled Tour in 2009, or zero to hero with a 2nd place at the 2011 Vuelta, the question comes back to: have we seen this meteoric rise before, in someone impartial to the current climate and team, who might be believable?
There are similar rises (Riis ~= Wiggins GT trajectory, for example), where the precedent provides evidence that it can be done; it's just that it was done by a doper.
We can leap right into, "advances in training and science" later ;-)