Draw a long, straight line along a ruler or straightedge. Draw a second line parallel to it. The easiest way to draw two perfectly parallel lines is to draw the second line along the other side of the ruler before you remove it.
Draw a third straight line, again along a ruler, but this time intersecting both of the parallel lines. Any angle will do, as long as it crosses both of the first two lines. This line is called a transversal.
Measure the angles at which the transverse line intersects the two parallel lines. If they are truly parallel and all three lines are straight, the corresponding angles of the two intersections will be equal.