Just an old Stanley 8" square.
Check a square 1.JPG
The 2 lines are from a test. Edge closest to the camera had been jointed a while back..so
Check a square 2.JPG
Use the straight edge, mark a line across the face...then, flip over..
Check a square 3.JPG
Make a second line beside the first...and see if they are parallel to each other...
Looks close enough for the girls I run with....
Try it sometime...you might like it