Smell a rat begin to suspect trickery or deception.
smell blood discern weakness or vulnerability in an opponent.
smell of the lamp show signs of laborious study and effort; the reference is to an oil-lamp, and according to Plutarch the criticism was once made of the work of Demosthenes, ‘His impromptus smell of the lamp’, meaning that his speeches were written rather than spoken orations.
smell the roses in North American usage, enjoy or appreciate what is often ignored.
See also fish and guests smell after three days, money has no smell, wake up and smell the coffee.