on guard

put sb on (one's) guard — настораживать; насторожить

be on (one's) guard (be careful and aware because a situation might be dangerous) — ~ держать ухо востро; быть настороже; сохранять бдительность;

Example 1: Something in her tone put Brian on his guard. — Что-то в ее интонации насторожило Брайана.

Example 2: "You're going to have to keep careful watch." Brian smiled. "You bet," he said. "And thank you, sir." "I mean it," said Damon. "Be on your guard." (Judgment Day,
Southern Review)

Example 3: “Those were to have been my problems,” I said. “Now I fear they will be yours. And you must be on your guard, my friend. The Klan's active hereabouts - you're safe enough in daytime, but a prominent colored man with a white wife should beware the night.” (The Overseer, Fantasy & Science Fiction)

Example 4: With Lou, the problem was quite opposite. He was a Lawrence scholar, one of those naturally timid bookish types who think they can transform themselves into earthy he-men by talking continually about sex and women. Butros and I started calling him "Lewd" between ourselves and then had to be on constant guard not to say this to his face. We liked being on our guard.. (Judith Caesar, Rotation)