I picked up a magnetic digital level a while back. I dont trust it absolutely, but its been useful for determining things like whether a head tube and seat are parallel enough to use as a reference to check the drop outs alignment.

Another check you could try is using your frame gauge to check the wheels alignment to the head and seat tube, in a similar manor to checking a derailleur hanger. If you trust the head and seat tube alignment, this would give better references than a tape measure between the rim and the stays and eyeballing it.