Also, 
keep a sharp lookout for. Be watchful for something or someone, as in 
Keep an eye out for the potholes in the road, or 
They told him to keep a sharp lookout for the police. The first expression, sometimes amplified to 
keep a sharp eye out for, dates from the late 1800s, the variant from the mid-1700s. Also see 
have one's eye on, def. 1; 
keep a weather eye; 
keep one's eyes open; 
look out.