to prove one's identity, to state one's credentials, to show one's ID, to present papers (to identify oneself to prove that one has the right to enter a place)
“Colette, Duras, Beauvoir, Yourcenar, Sagan, toute une histoire de femmes auteurs qui toutes prennent soin de montrer patte blanche, de rassurer les hommes, de s'excuser d'écrire en répétant, combien elles les aiment, les respectent, les chérissent, et ne veulent surtout pas – quoi qu'elles écrivent — trop foutre le bordel.” — Colette, Duras, Beauvoir, Yourcenar, Sagan —a whole history of female writers who all took care to prove their harmlessness, to reassure men, to beg pardon for the act of writing by repeating how much they love, respect and cherish men, and most of all don’t want — whatever they might write — to create too much trouble.
Literally, “to show white paw”. From the French fable Le Loup, la Chèvre et le Chevreau by Jean de La Fontaine, where the biquet (“kid”) asks a visitor at the door [the wolf] to show his white paw.