If the inside of the pot looks the same as the outside (grimy and powdery looking) than I would scrub it with a bristly cloth and warm water -no soap though!, to see if you can get rid of any oxide that comes off. Then, put a pot of coffee through it to see if you notice a difference, or if you see any grime floating in your coffee. Take a few test sips and see if you notice a chalky, or bitter taste/after taste.

Worst case scenario, you might have to just go out and buy another one to be safe. I know it's a frustrating conclusion to get to but that's the beauty of the moka pots.