There is no such thing as physical evidence from a legal point of view.
There is only expert opinion about physical evidence and expert opinion is an exception to the heresay rule.
The gold standard is always a confession and the next best is eyewitness testimony which is neither anecdotal nor heresay.
Originally Posted by NonP
I meant to say anecdotal/hearsay evidence. Anyway my point was that physical evidence should be required before doing something so drastic, though even this isn't so foolproof. (See the Contador case, where even the CAS themselves admitted that his intake of clenbuterol was negligible but proceeded to ban him anyway.)