1. The minimum age
Most adults-only resorts set the minimum age at 18. Some, such as The Club Barbados, accept guests from 16. If you are travelling with older teenagers, check the resort's own policy before you book.
2. Whether all-inclusive is the standard plan
At some resorts all-inclusive is the only plan. At others it is one rate among several, and the cheapest price shown is often room only or half board. Each resort page on this site says which it is, under "All-inclusive".
3. Whether the resort shares its grounds
Some adults-only resorts sit next to a family resort from the same company. Guests may share a beach, a lobby or some restaurants. Where we found this, the resort page says so.
4. The beach
Not every beachfront resort has a sea that is safe for swimming. Some resorts use a beach club reached by shuttle instead. Each resort page has a "Swimmable beach" fact where we could confirm it.
5. Opening dates and the season
Mediterranean resorts often close in winter, and Caribbean resorts sometimes close for repairs after storms. Check that the resort is open for your dates. Where we know of a closure, the resort page shows a note in the overview.