Bronica (medium format RF) had a gizmo with two synchronized polarizers, one on the lens, the other on the VF, so you could tell when you had reached the highest polarization.
That's with the Bronica RF... only.
For other, more modest cameras, you see through the filter, turn it to the high polarization point, use the triangle mark as reference (or the brand name or any other mark on the circular frame) and try to adjust it that way when you put the PL on the camera lens.
Another way... turn the polarizer at the same time you look through the viewfinder and pay attention to your meter. The moment the meter shows change (underexposure indicator comes up), you have your polarized view.
Someone mentioned once a gizmo for Leicas, but I haven't found or seen it. Actually, instead of a polarizer, I just bought a ND filter for my Leica. The general effect is the same, though I cannot increase saturation of the sky or eliminate reflections...
but I can live with that!