Never be too critical of anything you have received as a gift.
The condition of a horse’s teeth is a good guide to its age. If a man needs a horse and is offered one for nothing, he should not examine its teeth too closely before accepting it.
Here is an example of how the proverb is used:
‘My uncle has bought himself a new car and has given me his old one, but it rattles like mad and won’t do more than forty-five.’
‘It’s better than no car at all, though, isn’t it?’
‘Then don’t look a gift horse in a mouth.’