A check done by men during foreplay to see if a chick is clean enough to eat out. The guy fingers the chick's pussy and then he gets his fingers back into range of his nose to sniff without the chick noticing. It is easily done behind her back while kissing the side of her neck
Porn stars do not do the Smell-Check; 'Regualr' guys do.
-Having enough suspicion of a situation or event to the point where you have to investigate further to see if it was wrong or bogus (or BS).
-Hearing what seems like bullshit, and checking it out further.
"Tom said he couldn't come in today because he had an appointment."
"Hmm, that's funny. He didn't mention anything to me. When he gets back, I want to see his appointment slip to smell check his story."