SOme apps never delete data from the sd card when you uninstall them. So the first thing you can look for is folders from apps that you dont have anymore, those are safe to delete. Other than that, just look into each folder. If there is nothing there, its safe to delete. If you accidently delete any folder that an app is using, dont worry, the app should re-create that folder the next time it opens. However you will not get back any data that was in that foler.
