The answer depends on why you need to do that. For example
If you need to check the entities to which your own user's organization has access to, then just do a Resources["SearchFunctions"].SearchThings(), or a wider SpotlightSearch. You will only see what you are allowed to see.
If you are Administrator, and want to check to which org a Thing belongs to, you can iterate over all things in the system and do Things[name].GetVisibilityPermissions() to check the org.
If you do it for some security purposes, e.g. want to check overall visibility, then it becomes much harder, because then you need to account for the visibility inheritance through Collections, ThingTemplates, etc.
