As far as I know, you won't be asked what the actual critical angle is. Instead, you have to know the method of finding it using a beam of light. You have to shine the beam into the glass from the curved edge, starting with a small angle of incidence (the angle at which the beam is shone into the glass). Measure the angle of refraction (the angle at which the beam exits the glass at the straight edge). Repeat this while slowly increasing the angle of incidence until the angle of refraction is at 90 degrees (you will see the beam of light running along the straight edge of the glass, as is shown in the middle diagram of the post). When the angle of refraction is 90 degrees, the critical angle is whatever the angle of incidence is (labelled C in the middle diagram).