Basically there are 3 ways
1. Leica has a polarising filter, which swings in and out. So you swing it out to the Viewfinder part, adjust and swing it it over the lens. Pretty expensive filter
2. If the lightmeter of the camera reads TTL, then you turn the polariser; and at maximum polarisation, the time is slowest ( and vice versa); so you can approximate, but you dont see the result
3. In (at least some) polarising filter, there is a zero mark. So you put the filter to the eyes, turn to see the effect, note how far its off from the zero mark; put the filter back on lens; set it by the same distance from the zero mark and shoot.
In other words, solution 1 is elegant, the other 2 are hit and miss.