[gudgeonpin]

An old motorcycle trick is to use 8ft fluorescent bulbs to check for front/rear wheel alignment.  Don't see why you couldn't do something similar here-
Put a bulb parallel to each tire (touching rim or sidewall?) and centered, then measure the distance at the 'front' of the tube and the 'back' of the tube.  If parallel, the distances should be the same.  
I have no idea if that would work in practice.  I'm just making sh*t up as I go....