Осла знать по ушам, медведя - по когтям, а дурака - по речам

Folly and ignorance are betrayed by talking. See Дурак сам скажется (Д), Знать птицу по перьям, а молодца по речам (3), Обед узнают по кушанью, а ум по слушанию (O)

Var.: Осла узнают по ушам, а глупца по речам

Cf: An ass is known by his ears (Br.). Cracked pipkins are discovered by their sound (Br.). A fool is known y his conversation (speech) (Am.). You can tell a bad penny by its ring (Am.)