eye (someone) up
Definition: to look at someone carefully because you find him or her sexually attractive. (informal)
E.g.1: He sat in the bar eyeing up the woman who was sitting in the corner.
E.g.2: Do you think she likes me? Well, she’s been eyeing you up for the last half hour, so I think the answer is yes!
This phrasal verb can be separated. (E.g. 2)