Maybe I missed something, but I've not seen anything that proves she was a "covert" agent at all. References I've seen indicate that she was an analyst/researcher for the CIA working on WMD, not an foriegn agent working under cover for the CIA.
You won't see the proof that she was a covert agent because the government does not make that public. But the fact that CIA Director Tenet asked for an investigation, and the fact that we have gotten this far in that investigation implies that she was at least at one time a covert agent. It would be very easy for Tenet to determine that, and he certainly would not have asked for the investigation if she had not been.