A mask fitting should begin dry and continue in controlled water. Check the skirt seal without over-tightening the strap. Breathe normally through the regulator. Look down, across, and toward your buddy. The mask should remain comfortable while you make ordinary head movements.
Read the gauge at its normal position. Then check a computer screen, slate, and hand signals. Do not test only a large printed letter. Diving uses small information at changing distances. If you need readers, test the actual reading area before committing to full-lens correction.
The how to choose a prescription scuba mask guide can help frame that conversation. Ask the fitter to explain lens type, field of view, bridge comfort, and strap adjustment. Ask whether the mask can be serviced or replaced locally.
Practise the mask skills required by your course. A contact-lens user may worry about opening the eyes. A prescription-mask user may worry about seeing after removal. Either concern belongs in the briefing. A controlled, supervised practice session is better than discovering the problem offshore.
Stop the test when you have pain, repeated leakage, dizziness, or blurred vision. Equipment fit cannot diagnose an eye condition. Seek an eye-care professional for persistent symptoms. Seek qualified medical advice for any diving-suitability question.