Please read this:
Q. Do expensive sunglasses provide better UV protection than inexpensive sunglasses?
A. Not necessarily. While most brand name and designer sunglasses provide 100 percent UV protection, not all do. A professional optician usually can verify how much UV protection your sunglasses provide. All polarized sunglasses block 100 percent UV light.
http://www.opticaladvisor.com/eyeglasses.html
BTW, most of the optical stores have equipments to test if your sunglasses have good UV protection. you can just stop by and ask them to test the sunglasses for you.
