You should research the product you are buying. Contact the company. Checkout what the chemicals are added, there is no industry standard for which chemicals are added, it's up to the company. You can look up the chemical added on the internet for allergic or toxic reactions. Personally I have too many allergies to consider UVA/UVB clothing, not only to chemicals but to the fibers in those stretchy sun cover coverups. If I wear a cover up it's a tight weave cotton. Yes if it gets wet it's heavy, but it's also cooling when wet. Glamourous it is not.

I have to wear this kind of shirt if I am going to be outside for any length of time, so this question of yours interested me too. I did just a brief online search and didn't find much except this:
"Sometimes, companies add chemicals called “colorless dyes” to the fabric to increase its UV protection."
So, I guess you try to determine if the clothing has "colorless dyes" added to it.