When you want to get really precise you can use a semi-silvered mirror at one position and a front-
surfaced mirror on the other. That's how I leveled all the relevant surfaces with my big color enlarger, using an expensive industrial laser. The beam goes back and forth like a collimator and can be corrected within tiny fractions of a degree. No view camera can even be adjusted that closely.
But more down to earth, I checked out my selection of pendulum angle readers, and the highest
quality one short of a true machinist's tool is made by Tajima. A bit bulky for a hiking pack, but would
be excellent in a studio or for architectural work to check verticals.