daf bit: Kiddushin 76

[cellio]

The mishna teaches that before marrying, a man should investigate his prospective wife's lineage to verify her status. (The mishna has previously discussed categories of people who are and are not allowed to intermarry.) For a daughter of a priest he looks back two generations; for Levites and other Israelites, three. (The mishna is more complicated, but that's what it boils down to.) However, if the investigation reaches a priest who served at the altar in the temple, a Levite who sang in the temple choir, or a judge or public officer, he can stop there -- they've already been vetted.

The g'mara, however, says that this investigation isn't required: families are presumed to be fit, and we only investigate if there is some question. This mishna, the g'mara says, is about cases where the matter is contested. (76a)