The Instructor of Record in your course probably has a protocol for ensuring academic integrity during exams. Your first resource for preventing cheating on exams is the faculty member in charge. Make sure your students are aware of the faculty member’s rules.
That said, here are some principles for proctoring exams. Detecting cheating on an exam is mostly a matter of vigilance. Walk around the room and watch for wandering eyes, cell phones or other devices on desks, open books or book bags at students’ feet or on a nearby chair, or students who stop writing when you walk near them. Pay particular attention to students who sit in a part of the room that is difficult to see, such as the very back row or a dark corner. Consider seating students randomly in alternating chairs to minimize chances of copying or being copied from. The faculty member may provide multiple versions of the exam with items in a different order to minimize copying.
All cell phones and other small electronic devices should be turned off and put away. Sometimes students violate this protocol without even realizing it, because using these devices is such a powerful habit. Often students will use their phone simply to keep track of the time, or assume it’s okay to take a phone call during the exam. Remind them to turn off their electronics before the exam starts, and assure them that you will keep track of the time for them by writing the time on the board at various intervals throughout the exam period.
If you do see a behavior that goes against the exam protocols, do not immediately accuse the student of cheating or trying to cheat. Instead, give the entire class a general reminder of the rule and make it clear that the instructors take the rules seriously for the protection of all students. Your Instructor of Record may have more specific ideas about how to handle this during and after an exam.
If you determine that a student very likely did cheat, you need to follow through to some kind of resolution. This is dealt with on the Teaching Guide page If You Encounter Academic Misconduct.