You can do that by simply measuring the main angle and then measuring each of the two angles. If you bisected the angle correctly, you will find that each of the two angles is equal to half the original.
You can measure the angle by following these steps:
1- Place the straightedge on the base of the angle.
2- Slide the protractor over it until the vertex of the angle is at the zero of the protractor.
3- Measure the angle.
