unbrokenchain, I used this in the past to check an aluminum frame...Get a ~ 6"length of all thread that'll fit into the drop outs and four nuts. Use the all thread to spread the drop outs to what the width would be with a hub and tighten. Place the frame on the floor and mark both sides of the bottom bracket tube and the insides of the drop outs. Using a square draw a straight line rearward for each side of the BB tube and then measure to that line from the drop out marks. They should be the same left and right.
I'm know there are better and more accurate methods so use this as a quick check.
Brad