Yes, to check the inside of the roof box, you will need to pull the T-Panel and remove the cover plate for the door wiring. Then, you can get one of those small inspection cameras (or your phone camera if it will fit) down in there and take some pics.
You may see something that looks like this:
IMG_0951.jpg
On this particular box, there was really no external evidence of any issues whatsoever. If your car spent any good amount of time outside during it's existence, this can easily happen.