There’s the string around the headtube, back to the dropouts thing but that’s more for alignment along the line of the bike, your damage is more likely to be headtube angle change. You could lay a straight edge along the tubes and see how flat it lies but, and it’s a fairly big but, you’d want to know how flat it lay before the crash because tubes can have a curve before they are even built into a frame.
There’s the string around the headtube, back to the dropouts thing but that’s more for alignment along the line of the bike, your damage is more likely to be headtube angle change. You could lay a straight edge along the tubes and see how flat it lies but, and it’s a fairly big but, you’d want to know how flat it lay before the crash because tubes can have a curve before they are even built into a frame.