The day before Contador was tested positive for Clenbuterol, he tested positive for plasticisers. Plasticisers is a chemical found in plastic IV bags. A test reveled that Contador had eight times the minimum of this chemical in his urine suggesting he had a blood transfusion. Contador continues to deny that he doped on the Tour.
In related news, Bernhard Kohl straight up said "that's it's not possible to win the Tour de France without it [doping]". Big words coming from a retired professional rider.