Great question!

So yeah, I think we can assume some in-transit dialogue. Following the rule of "take ten when it's not necessary to roll", we know that Oracle's Know: Arcana is quite beefy so she'd be able to wizard-info-dump-you a fair bit.

There's bad news and good news for what she knows -- the bad news of course being that very little is known for sure about doppelgangers due to their habit of blending in -- no definitive treaties of doppelganger society has ever been written, and some scholars were driven to paranoid madness, assuming that the shape changers might be anyone and everyone they met. However, some things are known. For one, they're understood to generally be arcane beings, touched innately with magic the way most elves or human/gnomish magi are. As such, they strongly dislike psions or people who emanate psionic energy and -- more importantly -- their supernatural ability to shape change is nullified in an anti-magic field. So, if there are any lurking in here, that field would reveal them.

Now, one problem you could find is that neither you (Oracle) nor the doppelgangers can now read minds given the AMF. Some of the cells are fairly heavily crowded. So if there was an imposter hiding behind some of the assembled bards, they could be physically out of sight, absent a really fantastic spot or listen check. One could assume that the other occupants in the cell would be willing to give the imposter up, but being within arms reach of that "person", they might be too terrified to speak up, for fear of immediate retribution.

(This is "meta" knowledge, so if you object to it I can avoid sharing tidbits like this, but any of the more novice, junior bards could be assumed to have a very low character level, so an errant retaliatory slam could likely be lethal).

Finally, when Oracle clarifies that supernatural abilities are silenced in an AMF, I'll give Damien the wherewithal to ask if if there are any non-arcane ways to change one's shape, given Oracle's earlier reveal that Kate's psionics would continue to function inside, unlike everyone else's powers. Oracle answers by clarifying that AMFs generally work against everything, except what she describes as "either psionic or extraordinary abilities, both of which are rare". She further goes on to elaborate that, "If you wanted to shut Kate and I off at the same time, you'd need a way to overlap an Anti-Psionic Field with an Anti-Magic Field. For technical reasons relating to the energy source each field requires, doing so is incredibly difficult, if not impossible. It would take an incredibly powerful Archmage or Master Psion to cover the same area in both wards."

When Damien then asked, "Is our Blood Mage fellow that level of talented? Oracle almost laughed out loud:

"No. No no no nonononono. Not him. The rogue blood mage is likely slinging an enormous amount of raw power, but his 'skill', if you could call it that, is sloppy at best, and terrifyingly slipshod at worst. There's no way he could layer an anti-psionic field over this area. So unless the Karlo's have a much, much better mage in their employ who they simply didn't bother to commission for the rest of their defenses, Kate will be just fine."