In the US, the FDA requires sunscreens to list the active ingredients on the top of the drug facts label.
Check the active ingredients -- these are what make the biggest difference in sunscreens.
Mainly: is there oxybenzone or octinoxate? These two chemical filters have been banned in Hawaii due to their bleaching of coral reefs.
Other concerns may be staining, in which case you're looking for avobenzone. It is highly reactive with bleach as well as sunlight.
If you're looking for a safe option, look for solely zinc oxide and titanium dioxide. These are the only two mineral UV filters and are the #1 and #2 rated filters due to their reflection of broad spectrum UV Radiation and stability.