Your eye doctor will do a comprehensive eye exam, and will use various instruments to measure how your eye focuses light.

Your visual acuity will be measured, where you will be asked to read letters on a distance chart to determine the clarity of your vision at a certain distances. You will also have the focusing power of your eyes tested with a series of lenses placed in front of your eyes. The curvature of your cornea will be measured with a keratometer, and corneal topography may be used to provide additional information about the shape of the surface of the cornea.With these tests, your eye doctor can diagnose astigmatism and will be able to determine the power of glasses or contact lenses you require for clear vision. He or she will also discuss other options for treatment, such as astigmatism surgery.

fonte: https://www.aao.org/eye-health/diseases/astigmatism-diagnosis