Select one of the trapezoid's sides, and draw a perpendicular line from one end of the side to the side opposite it with the straight edge of your ruler. If the perpendicular line doesn't intersect with the opposite side, then extend the opposite side with your ruler until it does.
Measure the angle created by the intersection of the perpendicular line and the opposite side with your protractor. If the angle is 90 degrees then the two sides separated by the perpendicular side are the trapezoid's bases; if the angle is not 90 degrees, then the two sides are its legs.
Measure the lengths of both legs with your ruler. With the exception of isosceles trapezoids, the legs will have different lengths.