byrd48 I've many times taken a long strait edge and laid it across the sidewalls of a tire and looked to see how it matched up with the front. And eyeballed it to the rest of the frame. I rest it on the rear tire though and just turn the front to see if it matches or can be made parallel.
It's helped somewhat on a few occasions to help me note when things are wonky. But some questions I've ask several times in various places have never been answered.
- Will or should the front and back wheels always be in the same plane?
- If they are in parallel planes, then how much separation is okay?
- What effect is produced as the planes get further apart?
Realize that what you are showing in the picture with the straight edge being fixed to the front wheel doesn't tell you if the rear wheel might be in a plane parallel to the front. If you fix in to the edge of the rear wheel, then you might find it is in a parallel plain and only out by perhaps the amount of dish and axle spacers that may or may not be correctly set on your rear wheel.