Bechorot 36 - Who Can Be Believed About a Blemish
When there is no Temple and a firstborn cannot be brought as a sacrifice, a Kohen stands to gain most when a firstborn has a blemish. Therefore, although a regular Jewish shepherd is believed to testify that a certain blemish occurred naturally and he did not cause it, a Kohen shepherd is not believed. Rabban Shimon ben Gamliel says that he is believed concerning some else's firstborn, but not about his own, and Rabbi Meir says that one suspected concerning something can neither judge nor testify about it. Each teacher explains his reasons to trust or not to trust in a given situation, and the hopes of gain or inner justifications that a liar may have. In addition, "Kohen shepherd" may refer to a shepherd who happens to be a Kohen, or alternatively to anyone who works for a Kohen as a shepherd. Thus the Talmud goes through the two sets of possible reasons.
A Kohen is believed to say, "I showed this firstborn to an expert, and he concluded that it is permanently blemished, so I can slaughter it" - because in matters that may be discovered people usually don't lie.
Art: James Riddel - A Goat With Her Kid