How do I know if my sunscreen is expired?

The first thing to check is the expiration date on the bottle. FDA says sunscreen products must have an expiration date unless stability testing shows they stay stable for at least three years. If a sunscreen has no expiration date, FDA says to consider it expired three years after purchase.

That is the clean rule. The practical rule is even simpler: if the product has been sitting in a hot car, has separated, smells odd, or feels different than it used to, replace it. Sun protection is one of those places where being cheap is false economy.

If you are buying a replacement, look for broad-spectrum protection and the SPF you actually need for your use case. A bottle that is within date and easy to reapply beats a fancier bottle you never use consistently.