How to find the critical angle in an experiment
A simple experiment that can show you the critical angle of glass into air is to use a hemi cylindrical glass block, a laser light and a protractor:
The light initially enters the glass hemi cylinder perpendicularly (at right angles) to the glass (point A) and because of this, no bending of the light occurs.
Up to the critical angle light will refract out through the back of the glass.
At the critical angle light will travel along the boundary. You can read off the angle this makes to provide you with the critical angle.
Beyond the critical angle, total internal reflection occurs. This same experiment can be tried with a similar hemi cylindrical dish which you can fill with water.