een gegeven paard niet in de bek kijken

Etymology
Literally, “not to look a given horse in the mouth”.

Verb

 * 1)  don't look a gift horse in the mouth

Usage notes
The standard form of the idiom uses the rude or colloquial word, even though the more polite synonym is prescribed in general usage for the mouth of a horse. This has given rise to the rare alternative form.