2,235. A Kohein of Confirmed Lineage

Hilchos Issurei Biah 20:2

A kohein of confirmed lineage means one about whom two witnesses testify that he is a kohein, the son of person X the kohein, descended from person Y the kohein, reaching back to someone whose lineage requires no confirmation because he served as a kohein at the altar. If the Sanhedrin hadn't investigated him and found him to be a kohein, they never would have permitted him to perform the Temple service. Accordingly, we don’t investigate anyone who served at the altar or on the Sanhedrin, since only kohanim, Leviim and Yisroelim of fit lineage were appointed to the Sanhedrin.

Hilchos Issurei Biah 20:3

Nowadays, challah doesn’t have the status of a Biblical law, not even in Israel, as per Numbers 15:18, “When you come into the land...,” i.e., when all of you are in the land and not just some of you. When Ezra came to Israel, the people as a whole didn’t go with him. Similarly, trumah nowadays is a rabbinic law. Therefore it is eaten by today’s kohanim, whose lineage is based only on legal presumption.