Fig.: to become aware of someone else's hidden intention

I'd like to give her a DVD for her birthday, but I don't want to ask her what's her favorite movie, or she could smell a rat and understand my intentions

Vorrei regalarle un DVD per il suo compleanno, ma ho paura che se le chiedo qual è il suo film preferito mangi la foglia e capisca tutto

When little Joey began to clean his room without being told, his mother smelled a rat and thought there was something else behind

Quando Giacomino si è messo a pulire la sua stanza senza che nessuno glielo dicesse, la madre ha mangiato la foglia e ha pensato subito che ci fosse sotto qualcosa

Agnes pretended to have a lot to study every time it was her turn to do the dishes, until Mother got wise and made her do the dishes anyway

Agnese diceva di avere un sacco da studiare ogni volta che toccava a lei lavare i piatti, finché la madre ha mangiato la foglia e glieli ha fatti lavare lo stesso

When households rats were much more common than today, they would sometimes die inside a wall or under the floor and soon the householders would literally smell a rat. The rats we smell today are usually figurative - but smelling them still means that something is rotten. We don't know whether this or any other everyday image is at the origin of the saying, but we know that it has been in use since at least the mid-16th century, as a ballad from 1533 includes the refrain: “For yf they smell a ratt”