don’t look a gift horse in the mouth

Accept a gift in good faith. This saying, which dates from St. Jerome’s biblical commentary (c. A.D. 420) on St. Paul’s Epistle to the Ephesians, is based on the fact that a horse’s age is revealed by its teeth. Looking inside a horse’s mouth therefore will tell you if someone is passing off an old nag for a spry colt. The same expression is found in French, Italian, Portuguese, and other languages.