If you're shopping for sunglasses and aren't sure if they're polarised, you can easily check by holding two identical pairs at 90 degree angles to each other.
We've talked about why polarized lenses are awesome, but if you're browsing for glasses at a kiosk or market, you might want to double check the authenticity of their polarised selections. As the video above demonstrates, all you have to do is hold two identical pairs in such a way that you can see straight through a lens on each. As you turn one pair slowly to 90 degrees, your vision through the second lens should get visibly darker. If the lenses aren't actually polarised, you shouldn't notice a change.
You could also use this trick with most LCD smartphone screens. Just look at your phone in portrait orientation, then slowly turn it to landscape to see if you notice any change. If you can though, try to test this with a pair of glasses that you know are polarised beforehand, since not every phone screen behaves the same way.