1
general::
Uncover, as If there's anything to that rumor, Gladys will sniff it out. This expression alludes to an animal sniffing for prey. [First half of 1900s]
American Heritage Idioms
2
general::
Phrase(s): sniff someone or something out
to locate someone or something by sniffing or as if by sniffing. • The dog sniffed the intruder out and the police captured him. • The dog sniffed out the mole in the lawn.
McGrawhill's American Idioms And Phrasal Verbs