Your best bet for non-congested worlds would be to check them during the non-peak hours—usually late-late nights or early-early mornings when there aren’t a lot of people currently logged in, or immediately after a maintenance. Worlds lock themselves to character creation when there are a lot of people currently logged in. For congested worlds, there’s no way of knowing when the developers will unlock them. Best bet is to check whenever they do patches/maintenance—they usually announce when they make changes to Congested/Preferred worlds.