Tues Gitten 2

The gemara says that we only believe an individual witness for an issur if it' doesn't have a Chezkas Issur.

Tosfos explains this to be, only when it's not within your hands to make the Issur permitted. However, if it's your hand , then he's believed. If this is not true, nobody would believe that his meat was Shechted correctly, or his produce was already tithed. We even believe a woman that the meat is Kosher, even though she cannot Shecht. Either, because it's in her power to learn how to Shecht, or it's in her power to hire an expert to Shecht.