Step 1: Find some other method to summon a Planetar and render it bound/helpless.
Step 2: Make it put on and take off a Helm of Opposite Alignment 'til it rolls a nat 1 and its alignment changes to Evil.
Step 3: Ask it its name.

You probably don't have to worry about it being reversed, as (from the SRD):

Only a wish or a miracle can restore former alignment, and the affected individual does not make any attempt to return to the former alignment. (In fact, he views the prospect with horror and avoids it in any way possible.)