montrer patte blanche

To prove yourself trustworthy enough to get in

Used for anything from flashing an ID card to meeting whatever criteria let you through the door.

Pour entrer dans ce club privé, il faut montrer patte blanche.

To get into this private club, you have to prove you're worth letting in.

Comes straight from La Fontaine's fable about the wolf, the goat and her kid. The mother goat tells her kid to let no one in without a white paw as proof, and the wolf, of course, doesn't have one.