keep a weather eye out
An instruction to monitor someone or something, often because the person or thing in question may turn out to be dangerous. I think Tom's a thief, so keep a weather eye out around him. I'd keep a weather eye out with this storm that's brewing—we may have to evacuate.
keep a weather eye out
Also, keep a weather eye on or open . Be extremely watchful or alert, as in We should keep a weather eye on our competitors in case they start a price war. The precise allusion in this expression is disputed, but presumably it refers to watching for a storm. [Early 1800s]