Ces (these) and ses (his, her, its) are homophones, so no one would ever know if you mixed them up when speaking. Writing, however, is another matter. Whether out of ignorance or carelessness, it's easy* to confuse ces and ses - but it's just as easy to figure out which one is correct. All you have to do is think about what you'd say if the noun were singular, since the singular adjectives are not homophones.
For example: Il a perdu (ces/ses ?) clés. While both possibilities are grammatically and logically correct, which one to use depends on what you're trying to express. If clé were singular, would you use cette (the singular demonstrative adjective) or sa (the singular possessive adjective)? The answer to that makes it crystal clear whether you want to write ces (plural demonstrative) or ses (plural possessive):
Il a perdu cette clé (He lost this key)
> Il a perdu ces clés (He lost these keys)
Il a perdu sa clé (He lost his key)
> Il a perdu ses clés (He lost his keys)
Confusing French pairs
*Even native speakers sometimes mix up homophones in writing - how many times have you seen its and it's or to and too used incorrectly by English speakers?