2 options:
very obvious one is check if security seal is broken - if not, theres no way its modded. If the seals gone:
(on slims its on the back left side and quite square. On the fatboys its on the left side where the bottom, top, and backplate meet.)
-tear the console apart to see if you see the chip.
-burn some media (small homebrew game, bootdisk, backup, etc) and see if it loads.
Depending on the chip it may be possible to try to enter its config menu, or disable it.. hard part is, every chip works differently.
If you want I can check for you, and if its not I can pop one in if thats what you wish
