Chullin 12 - Most Who Slaughter Know the Laws
Rav Dimi bar Yosef inquired of Rav Nachman, "If one told his agent to go and slaughter an animal, went to the animal and found it slaughtered, what's the law?" Said Rav Nachman, "He can presume that the messenger did it, and the meat is kosher."
Rav Dimi then asked, "If one told his agent to go and separate terumah, the kohen's portion, then went to his produce and found the terumah separated, what is the law?" Rav Nachman answered, "He cannot presume that this was done properly."
Asked Rav Dimi, what is the difference between the two cases? If one can rely on an agent to accomplish his mission, he should rely on his in all cases. Said Rav Nachman, "When you measure me a kor of salt (about 25 pounds) of salt as my commission, I will explain." In fact, he cannot presume that his agent did what he was told to do. However, in the case of slaughter, even if another man did it, most who slaughter know the laws. In contrast, the separation of the kohen's portion requires the knowledge of the owner.
Art: Winslow Homer - Salt Kettle - Bermuda