I saw a great high school science fair experiment where a person was testing different brands of sunscreen. The girl made bacterial cultures in petri dishes, then smeared half of the cover with sunscreen and baked them under a UV light. She then counted the remaining bacteria to determine how effective the UV blocking was.
That's probably more complicated than what you are looking for.
You could check the brand name and search Google for it, or check at your local sunglasses vendor.