You can never really be certain about the measure of an angle without using a protractor, or some similar tool.
Usually, you can try to "eyeball" it, though. Just remember that a right angle is the same as an angle in a square. It is also a quarter of a whole revolution.
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by using a protractor
Using a protractor is the simplest way of solving the measurements of angles.
Using a protractor helps
Angles are measured using a protractor
Using the formula: (number of sides -2)*180 = sum of interior angles