If you place your fingers over the two lines on the angle, thereby making a triangle, you can note the length of the 3rd side created by your finger. If you do the same for another angle, and make sure to place your finger the same distance away from the apex, you can discriminate between the two angles just on the length of your finger that fits between the two lines.
This might work on a computer that has a high resolution and has a flat screen, but most of the computers at my center were older and had curved screens and poor resolution. So be careful when you study to make sure you can do the angle section withput counting pixels at the base of the angle or putting your hand to the computer screen.