You can see any and all transactions.
Any bitcoin in/out of Mt.Gox has to be verified by the network...a "fake" bitcoin would never get out and it would not benefit Mt.Gox to transfer fake BTC around internally.
A bitcoin cannot be counterfeited easily...hence the beauty of the system.
People can claim that there was never 500k bitcoin in Mt.Gox but the fact still remains there was a transfer of 500k just days before this incident that was discussed as possibly being Mt.Gox moving their internal wallet around.
Now if someone controlled over 50% of the network hashing capability that would be a different story...possibly they could inject false info or something like that (I admit I know very little about this part, just briefly reviewed it enough to know its bad) but that's why you see postings in the forums telling people to switch "guilds" to spread things out. Deepbit approached 50% last month and there was a mass exodus. Most of the people involved in this know the risks and work very hard to self regulate.