If you have a good ruler and a good set square, you can do it. You put your set square in line with the side you're suspecting, put your ruler on the other side of the set square, slide the set square without moving the ruler to the other side. If the latter is dead along the set square, the sides are parallel.

NO,NO,NO!! When Euclid and I made the rules we only allowed you to use a straightedge and compass.

Hm... I think I know how you did. Mine is faster though :D