Verb.) The pre-curser to fucking a vagina whose quality and cleanliness is unknown by yourself and your friends. The quick sniff is the act of fingering the said vagina and, while the vagina's owner is unaware, swiftly moving those fingers past your nose as if you have an itch, to smell if it is safe to insert your penis. If an odor is detected, it is then justified for the fingerer to shout "I have to go, something is burning!" and then leave promptly. Permanently removes that vagina from the "Doable" list.
Guy 1: "Last night I was fingering that chick I met at the bar and did the quick sniff... So glad I did because the bitch smelled like raw sewage! I ran out of there like a mother fucker!"
Guy 2: "Thank God you told me that! I wanted to fuck her. Taking her off the list for good!"