Green’s Dictionary of Slang

1. to act as a police informer [SE snout, i.e. one who ‘pokes their nose in’].

1923		E. Wallace Missing Million 161: He owned up to being Tod because the gang found he was snouting .
1928		E. Wallace Elegant Edward 14: You get your living by what is vulgarly called by common people ‘snouting,’ and I don’t want to have anything to do with a snout!
1959		J. Gosling Ghost Squad 128: Honest crooks — men who hated snouting so much that they wouldn’t shop their most dangerous enemies.
1972		P.D. James An Unsuitable Job for a Woman 385: Bernie had explained that the Snout was a police informer and did rather well. [...] She suspected that Bernie too had done some snouting in his time.
1974		G.F. Newman You Flash Bastard 37: This kind of meeting was little more than a PR job for the DI; his reassurance kept his grasses snouting.

2. (also snout about, snout around) to search [SE snout, i.e. one who ‘roots around’].

1984		Partridge DSUE (8th edn) 1106/2: [...] later C.20.
1990		J. Ellroy ‘Gravy Train’ in Pronzini & Adrian Hard-Boiled (1995) 500: You got word that Curtiz was snouting around.