Avodah Zarah 40 - How to Tell a Kosher Fish
If a fish comes to us broken into pieces, then in order to ascertain that it is kosher, we need both the head and the spine to be discernible - so says Rav Huna, but Rav Nachman requires either the head or the spine.
The fatty innards of fish and their roe (caviar) may be bought only from an expert in identifying kosher fish, scrupulous in the laws of kashrut. If no expert is available, then if the Jewish seller says, "I personally salted the fish and saw that they were kosher fish, and it was from those very fish that I removed the innards, - he is believed, because were the innards taken to an expert and found to be non-kosher, he would have not way to defend himself. That is the opinion of Rabbi Yehudah.
Rav Nachman requires the seller to show the fish. That is because the seller might not be an expert and thus, without intentionally lying, still be mistaken. However, Rav Yehudah ruled in practice according to his view that as long as he says, "I personally salted the fish," the caviar is permitted, and ruling in practice determines the law.
Art: Frederic Bazille - Still Life with Fish