One very easy way to see the CPL effect is to look for a glass surface with a strong reflection, e.g. a window or a car windscreen. With the CPL on the lens, as a certain angle, the reflection will completely disappear, and the glass will appear to suddenly become transparent. Its like bloody magic.
This because glass will polarize light reflecting off it but will not polarize light going through it. So the CPL will block the polarized reflected light but not the light shining through it.
Light from certain angles of the sky will be strongly polarized, so CPL will be more effective. Usually CPL is nearly useless when pointing towards the sun, and most effective when pointing at a 90 degree angle from the sun. (please don't ask me to explain the full theory behind this, I only understand the general idea lolol)
Have to use trial and error to find the best angle, it doesn't always work all the time!
