Умному свистни, а он уже смыслит

Give a covert suggestion to a clever man and he knows what to say or to do. See Одному глазом мигни, а другого дубиной толкни (O), Умный понимает с полуслова (У)

Var.: Умному - намёк, глупому - толчок

Cf: Half a word is enough for a wise man (Br.). A hint is as good as a kick (Am.). A nod for a wise man, and a rod for a fool (Br.). A nod is as good as a wink (Br.). Send a wise man on an errand and say nothing to him (Br.). То one who understands, few words are needed (Am.). A word is enough to the wise (Am., Br.). A word to the wise (Br.). A word to the wise is sufficient (Am.)