Kiddushin 76 - Checking the Family Line

A Kohen who wishes to marry a daughter of a Kohen must examine the status of her ancestors: her mother and the mother of her mother, the mother of her mother's father and her mother, the mother of her father and her mother, the mother of her father's father and her mother - but only if there is some evidence of a blemish.

If in that line he finds a Kohen who served at the Altar, or a member of the Sanhedrin, or a public official, or a charity collector, the investigation stops. The latter two are often involved in quarrels, so any blemish in their genealogy is already known.