The first thing they're going to try is actually Reduce Person, on a random two of them. It doesn't work on demons, and it can be cast from behind the walls such that it'd take arcane sight and good spellcraft to guess what it is and fake it (and take more skill than that to fake it without any visible trace).

(Also Discern Lies but that shouldn't be noticeable at all.)

Now Kevin is about three feet tall! "Ack!" He's already constantly scanning for mals demons out of habit and there aren't any so it's probably a side effect of the attempt to prove they're human.

And Marcy is even shorter than that! "Wha--I presume that was part of the plan."

Hopefully he wasn't also supposed to shrink; he doesn't think he did anything that would prevent it.

And if they're demons they should have an ever harder time appropriately timing the dismissal of the spell, which the wizard through the wall will do at a time not known to her in the next few moments. "I regret the inconvenience," she says politely. "The spell doesn't work on demons, and it's among the simplest that doesn't. It takes about five minutes to wear off and will not have any effects once it is gone. Can I ask for the rest of you to give me your own accounts of what happened here?"

"By here so you mean since we arrived?" She and Kevin and Franklin can take turns recounting the events of the past hour or so, interrupting each other to add details. Between the three of them it's a very thorough account.

Shortly into it those who were resized return to their original sizes. She listens patiently. Thanks them.

Pulls out and opens a box with a bunch of glass vials of liquids in them. "Do any of these stand out to the new sense that you received unexpectedly?"

Staaaare. "Yeah, there's something in there that registers."

Several more seconds of staring. "The green one third from the left."

"I agree."

"All right. I'm going to cast a spell that is seriously injurious to people who are not Good. You all appear to be Good, so the spell shouldn't affect you, but if any of you deliberately used magic to make yourself look like you're Good when you aren't, this could kill you, so I'd rather you tell me now."

"Not on purpose but I don't know how our kinds of magic interact, and the gate should have dispelled everything on us but we already know it malfunctioned, so can we have a second to counterspell ourselves first and see if we still read Good?"

"Of course, go ahead."

They make sure their persons are clear of any lingering enhancements and put some of their copious jewelry (not their power-sharers) in a little pile on the ground. "Okay, if we still read Good then go ahead."

She pulls out a scroll of heightened Holy Word. 

She is not, to be clear, going to use the scroll of heightened Holy Word. It is ludicrously expensive. If one can't check whether an apparent lost adventuring party is an actual lost adventuring party or some disguised demons using exclusively first and second circle spells readily available to a Worldwound fort, one has no business being an inquisitor at a Worldwound fort. But demons relying on the fact that no one at this particular fort is above fourth circle might be a little nervous, and Lawful Good sorcerers from another world will not be able to tell the difference at all between this and the spell she actually has cast, Fear the Sun, which blinds everyone it affects be they Good or Evil. A demon blinded by Holy Word would probably try to fake not being blinded; a Good sorcerer would probably just be genuinely confused, or assume the spell does that to everybody.

Aaaaaaaaaaaaaaaaaaa

"ohfuckohfuckohfuck"

"I--can't see--"

Shitshitshit something went wrong and they're all gonna die

Aaaaaaaaaaaaaaaaaaa no stop get it together, you're everyone's best chance, what's the right counterspell for this--

And six seconds later it wears off and they switch from shaking in terror to shaking in relief. 

"I'm okay, is everyone okay? Uh, temporary blindness, if you're wondering what happened."

They all assert their okayness in variously shaky voices. 

(They all remember that kid who got the cursed magnifying lens from home. Memories are all that's been left of him for a long time now.)

"I'm sorry we don't have a better method of verification," she says. "All of our tests were consistent with what you're saying, and you are welcome to come into the fortress now. I am sure we have lots more questions for one another but it would be very understandable if you wanted a meal and a rest first." 

Oh good, this place is enough like the Scholomance that they're not bothered by the ways people come out fucked up. They pull themselves together and get their artifice back on.

"Any reason not to eat and swap questions at the same time? Starting with, I thought that spell was going to either do nothing or be permanent; is it being temporary a thing that's known to happen or did it interact badly with our magic in some way?"

"No, I just lied to you about what reaction we expected because it would have been easy to fake the outcome of the test if you'd known what it was supposed to be. Most people here have oaths that preclude lying to their allies in any context; I am permitted to, specifically with respect to what outcome on a test counts as a passing one. The spell that I appeared to be casting, to someone who knows Golarion magic, has no effects on Good people and temporarily blinds Evil ones too strong for it to kill; the one I actually cast temporarily blinds everyone.

An Evil person who knows how our magic works would have expected to be blinded, and likely tried to resist the spell or if that failed pretend that they weren't.  I'm a little worried about bad magic system interactions, even though everything so far has functioned as you'd expect it to, which is part of why I didn't want to actually use the spell that kills or blinds Evil people. Though the bigger consideration is that that's an expensive one and we save it for emergencies."

The gates open. "We can absolutely eat and swap questions at the same time. If you have truth magic of your own, we are willing to be made subject to it."