In class exams
If a student is taking a test or quiz in class, let them know that you can't help them with the exam, but if they come back later, you'd be happy to help them go over any problems that they missed so they're ready to move on to the new material or when they're studying to prepare for their next exam.
Take-home or online exams
Often, students working on take-home or online exams are allowed to work together or use outside resources for their test. Students are usually pretty honest about this if you ask them. We always encourage tutors to avoid doing the work for their students. This is especially important during exams. If your student needs more detailed help on a problem, try using a similar example to show them the steps so that they can work through their exam problem independently.