Besides the definition, what other way can you use the sides of a quadrilateral to prove that that it is a parallelogram? (Site 1)

A parallelogram has 4 sides, 2 of them are parallel to each other along with the others.

You can see how far (from the middle of the quadrilateral) is one end of the line and then indicate how far the other end is. If those numbers are equal to the other lines distance then they are parallel