More formal:
* Peripheral vision: This is the most accurate scientific term for the ability to see objects outside of your direct line of sight.
* Lateral vision: This term emphasizes the direction of the vision (to the side).
More informal:
* Sideways glance: This emphasizes the action of looking to the side.
* Out-of-the-corner-of-your-eye vision: This is a more descriptive phrase that conveys the feeling of seeing something peripherally.
The best word to use will depend on the specific situation and the desired level of formality.