Whlle looking at a lake or glass store front spin the filter until the reflections "magically" disappear in your viewfinder.
While looking at a sunny seen spin the filter and the sky will "magically" get darker or lighter. Stop when the sky looks the way you want it and snap the shutter. How dark the sky will get depends on where the sun is at the time. Of course you will see all this while looking through your viewfinder before you ever take the shot.
I'm telling you this because I think in a year from now you will be "kicking" yourself for not buying a polarizer sooner.