Squint, blink, narrow the eyes, look closely, peer, gaze intently, scrutinize, examine carefully, study, inspect, survey, observe, eye up, contemplate, eyeball, watch closely, spy, peek, and regard may all be used as synonyms for "screw up the eyes". These words all describe a deliberate or involuntary action in which a person tenses the muscles around the eyes in order to focus more clearly on something. While each of these words has its own connotations and nuances, they all convey the idea of close and careful observation. Whether you are examining a piece of art, scrutinizing an opponent, or simply trying to see something in dim light, there is a synonym for "screw up the eyes" that will fit your purpose.