Most—but not all—polarizers have an axis-of-polarization index mark fixed onto the independently-rotating ring which holds the polarizer in the non-rotating mount affixed to the lens. The response of post #2 is correct, with a proviso. This is particularly useful when using a polarizer on a TLR, rangefinder, or scale-focusing camera.
Some polarizers have been made with a short handle to make it more convenient to rotate the front ring. IF the handle has been set to the axis of polarization, then the handle or “arm” can be used visually to align the polarization axis with the sun.
This index mark is convenient when doing double polarizing setups in-studio. In this setup, the polarizing screens over the lights have the axis of polarization marked on their mounts. For proper effect, the index marks on the screens affixed to the lights must match the index marks on the polarizer affixed to the lens.
This is illustrated in the Kodak Book:
Copying and Duplicating in Black-and-White and Color
https://www.amazon.com/Copying-Duplicating-Black-White-publication/dp/0879853433
https://photo.stackexchange.com/que...le-indicator-on-my-circular-polarizing-filter
Other polarizers have no index mark and the effect must be determined by viewing thorough the filter and rotating the polarizer until the maximum effect is observed. When I get such a filter, I test the filter for the position of maximum effect and place an index mark on the rotating frame with a small dab of contrasting lacquer or nail polish on the rim towards the sun or other principal light source.
Post 4 is also correct, of course. It applies to any polarizer, those with axis index marks and those without.
The index mark is also be used as follows: Hold the polarizer in your hand while viewing the scene from the camera position and rotate it until the maximum effect is observed. Note the orientation of the index mark. Install the polarizer on the lens and rotate the polarizer so that the index mark is in the same orientation as when you viewed through it. This is yet another way to ensure maximum polarization. This is useful for TLR, rangefinder, scale-focus, and view cameras (in situations in which the view is rather dark).
