Wiktionary
vb. 1 To find something using the sense of smell. 2 (context idiomatic English) to find, especially to find something that cannot be seen.
WordNet
Usage examples of "sniff out".
The tribe needs someone who can sniff out the trail to its source.
Father Crosby, who can sniff out a rumour faster than a ferret can shoot down a rabbit hole, has already been to see us.
Its greatest talent, however, was its ability to sniff out arrangements and combinations that could result in new perfumes.
I needed her keen nose and extra perceptions to sniff out the demon.
Even most of the Aes Sedai were ignorant of that, but he knew how to sniff out things.
Their skills differed from Aes Sedai's -- less in some places, more in a few, but generally just different -- yet they should be able to sniff out any unwelcome gifts.
There was no danger that came to them that I, or Kai or Kayi, could not sniff out and give warning of.
If any scout tried to sniff out the reptile scent, he would be disheartened by these further precautions.
He eased around, trying to sniff out who, or what, might be back there, but the wind was out of the wrong quarter.
No requirement for these night-seeing devices then, we would know at a glance if the house was occupied, and if it was we'd soon sniff out the occupant.
His true purpose was to sniff out the Darkfriends and Trollocs if he could.
I thought we could conduct an experiment to see if a cat can sniff out bookworms.