They are also becoming popular with truck drivers and for general outdoor activities. The last five years have seen a jump in their sales and are the fastest growing segment in eyewear. Very easily done. Then, turn the sunglasses around as if they were the hands of a clock facing you. If the intensity of the reflection doesn't change with respect to what you see through the window, they are not polarized.
Normal sunglasses decrease the intensity of everything by the same amount.
Polarized sunglasses can selectively eliminate the reflection from light coming from above the water surface. Nope, they will always decrease the intensity of unpolarized light by at least half. In practice somewhat more because of losses in the material. First, if there is glare, they help a lot by reducing its blinding effect. But remember, it only works if you look at the water at some angle and not straight down.