When a person feels an itch in their butt crack and scrapes a couple of fingers across their asshole only to sniff them therefore confirming that they now have shit smelling fingers. Everybody in the world has done this or they are an outright liar!
Guy #1: Did you see Dennis scratching his ass? He just smelled his fingers! Guy #2: He does that all the time. He's doing a stink check. Guy #1: Gross...... he just handed me a beer!
When your Lover lets you go down on her and you do a "Stink Check" which is you sticking you fingers in their ass then proceding to twirl around ending it by pullin them out then sniffing and though is a stink check
"i stink checked my girl last night"