As the docs say:
When a user of your app specifies they want to use a file or a folder, the system adds the associated path to your app’s sandbox.
It also mentions that the sandbox includes "certain directories, including" a few, but there doesn't seem to be any authoritative list.
It also says:
Powerbox has no API.
Is there a way to get a list of all the files that are currently in my sandbox?
sadly no. you can/have to check each file/folder.