As a test of straightness, have a skinny pole placed between two poles of normal width and go down over them in different paces.
This, especially in an arena with a mirror at the end for you to check yourself in (so, no looking down, FEEL whether the horse is staying straight or not!) is a very good test of how straight your horse is, and how quick you are to correct with your legs if there is a wobble.
I put the poles close together for the photo, but you could of course have a much longer distance between them.
This tip was told to me by accredited B.E. Trainer, Tina Ure.