keep a weather eye open
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 open around him. I'd keep a weather eye open with this storm that's brewing—we may have to evacuate.
keep one's weather eye open
Fig. to watch for something (to happen); to be on the alert (for something); to be on guard. Some trouble is brewing. Keep your weather eye open. Try to be more alert. Learn to keep your weather eye open.