My brother left me access to his lvl 66 Mage while he is away for the summer. How do I find out what zones the Mage is flagged for? I am unable to ask him, is there a way to find out?

Take the mage in question to the seer in pok she is in the building near the plane of tranquility stone. IIRC!

Sit the mage down and hail her.

That will tell you about PoP flags only.
/key will give you some useful information too, including Veeshan's Peak, Sleeper's Tomb, Qvic, and Txevu.
As for Kod'taz, I think the only reliable way to check is to try zoning in.