APAD: Don't look a gift horse in the mouth.

You can tell the age of a horse from its teeth, but if it's a gift, just say thank you and don't try to find fault.

There are corner cases, e.g., when the gift might be a marketing ploy. In fact, sometimes one should do exactly the opposite. Troy wouldn't fall had they looked into the mouth (and maybe the esophage and stomach) of that famous horse when everyone believed it was a gift from some god.