If you have a small refractor one can use it. The flat is placed on the ground and starlight is reflected off of it into the small refractor. You observe the stars at high power and move the refractor from about 45 degree to the shallowest angle possible. The stars should stay round and tightly focused if the flat is flat. If the flat is larger then the objective in the refractor you need to do the test over a couple of different cords on the flat.

- Dave