As mentioned, looking through it is your best bet. Remember to adjust exposure appropriately (usually 1.5 to 2 stops) for the polarizer.

Quote Originally Posted by bsdunek View Post
I use Rol_Lei Nuts method. Most polarizers have an index mark on the rim that should point towards the sun for maximum effect.
Let me add that while useful, this comment taken by itself may be somewhat confusing. This does not mean that the polarizer itself will only work if aimed (on camera) at the sun. In fact, the exact opposite is the case (or perpendicular ). Polarizers are only effective in a range of angles at 90 degrees from the sun. In other words, and you can test this for yourself before ever mounting the polarizer on the camera by looking through the filter and rotating it, the light cannot be polarized effectively when the view is either into the sun or in a broad range 180º opposite of the sun. One time–honored, off–the–cuff, method to deciding whether a polarizer may be used effectively is to hold your hand up with fingers pointing towards the light source (i.e., sun) and outstretched thumb. Your thumb will then point to an approximate direction at which light may be polarized effectively by your filter.