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.