Using a ruler or a distance measure item and make sure the distance between them is always the same. At least in 2 points, it must be.
Step-by-step explanation:
Parallel lines are by definition the ones that never touch each other. So are two lines "pointing" in the same direction and the space between the two lines never change.
In order to confirm the best method is to compare at least 2 measure of the space between the lines and must be the same. This must done with the highest accuracy.
