Determine whether the two lines forming the angle are perpendicular to each other. Assign the value of 90 degrees to any such angle. As an example, each angle in a rectangle and square has a measure of 90 degrees.
Determine the value of an unknown adjacent or supplementary angle by subtracting the value of the known adjacent angle from 180 degrees.
Determine the value of one unknown angle in a triangle by subtracting the two known values of angles from 180 degrees. The value of all three angles in the triangle should add up to 180 degrees.
Determine whether the triangle is an equilateral triangle by measuring each side of the triangle. Assign a value of 60 degrees if all the sides of the triangle are equal to each other.
Determine the value of an angle within a circle from the percentage of the circle that the angled section represents. If the lines representing the angle constitute a quarter of the circle, the measure of the angle will be a quarter of 360 degrees or 90 degrees.