How do I know a cotton duvet cover is actually high quality and not just marketed that way?

Look for independent certification (OEKO-TEX Standard 100 is on most of the picks here, and a Good Housekeeping Seal is rarer and more meaningful), 100% long-staple cotton rather than blends, a stated weave (percale for crisp/cool, sateen for smooth/silky, waffle or washed cotton for textured), and corner ties plus a non-zipper closure. Cross-checking high-trust reviewers against verified-purchase wash-durability comments is the single best filter.