There is one shim in each lens. The taking lens' shim is very thin and silver, matching that lens' thread colour. The viewing lens' shim is thicker and is black, matching that lens' thread colour. See attached pics.
I'm pretty confident I have the ability to detect the misalignment. I have spooled tracing paper between the two take up reels, and use a thin sheet of perspex/Plexiglas (in place of the pressure plate) to hold the tracing paper flat.