To determine if two angles are supplementary, first take their degree measurements using a protractor. Then, add the two figures. If their sum equals 180 degrees (the measurement of a straight line), then they are considered supplementary angles.
Though angles do not have to be adjacent to be supplementary, because supplementary angles pair to make straight lines, they can be used to find the measurements of unknown, adjacent angles. Care must be taken, however, not to confuse their properties with those of complementary angles, which are closely related but pair to make right angles and add up to 90 degrees.