Originally Posted by LandSpeedRecord
Wrap a string around one rear drop out, loop it around the head tube back to the other drop out, making sure they are laying against the drop outs identically. Then take measurements from the string to the seat tube on both sides, also attach levels to each drop out so they extend above and below by at least a foot then measure that it is true in that direction. If your local bike shop has one Park Tool makes a frame alignment gauge that you press against one side then set the contact point to the rear drop out then flip to the other side. Honestly you can be as or more accurate and thorough with string, levels, rulers, and a plump bob.
I didn't quite understand this the first time I read it. During a Google search, I came upon a Sheldon brown article where the exact same method is described and pictured. I then understood it and realized it was the same method you described. And upon measuring, I found the distance from the seat tube to the string to be equal on both sides, about 1.375". Thank you for your help, although I did not understand it at first, it was correct and I figured it out eventually lol.