I am fond of the cheap knockoff sunglasses because … well because I tend to lose sunglasses a lot. I’d like to think though that I’m not being penny wise and cornea foolish. Is there any way to independently verify claims of UV protection?
If you go to upscale toy stores or science kit catalogs (usually aimed at kids) you can get “blueprint” kits which contain photo paper with a blueprint emulsion on it. In fact, you can probably just buy the paper. This emulsion is sensitive to UV light.
Put a piece of the paper in the sun. Take your el cheapo sunglasses and put them on top of it. Expose for the suggested length of time (about 15 minutes, if I remember how to do this correctly). Take the paper inside and develop.
If the whole piece of paper is blue your sunglasses are a rip off. If there’s a big white splotch where your glasses were then they block UV. Intermediate states between dark blue and pure white indicate partial effectiveness.
It’s not calibrated or exact, but it will give you a rough idea of effectiveness.
If you lived in the UK, then it’s a leagal requirement for ALL sunglasses to offer 100% UV A+B protection, but I don’t know if it’s the same in the US
Can’t cite this, but take my word for it. 
Even the cheapest crappola sunglasses that offer UVA and UVB protection are legit.
I saw a Discovery Channel (???) story about how there are only one or two companies that make the protective UV sunglass layer, and they are the suppliers for everyone from Oakley, to NIke, Bolle’, RayBan to Joe’s Cheap Sunglasses, to the rack at the local pharmacy.
Thye tested the protection and they were all very similar.
One of the biggest factors in UV penetration was from the side. They emphasized getting glasses that wrapped around as much as possible. Many glasses had side leakage because they didn’t wrap. Luckily, many hip style glasses do wrap, and even the cheap imitations wrap. But, when it comes to the UV coating, they all were similar.
Make sure there is a UVA and UVB label.