Forget where I had read it, maybe I made it up, but I took the "The RHC has ways to confirm you are serious and good-hearted and dedicated to the RHC and Risur" as "It would be a neat scene if there were a lie-detector type questioning, with a dire penalty to scare away infiltrators" as a PC intro.
The part you are looking for is in the Player Handbook on the RHC.
"
Loyalty to Risur
The RHC recruits from police, military, universities, and many other sources of talent, occasionally even accepting foreign applicants. In addition to requiring extensive background checks, recruits must undergo a magical inquisition.
The king grants each branch’s local director the ability to test the loyalty of all who would apply to join the constabulary.
A candidate who agrees to undertake the test opens their mind so the director may sense their true intentions. If they have any ill will to Risur, its people, or its leaders, it will be revealed. More importantly,
the candidate must show a devotion to protecting Risur.
Risur need not be the
primary concern of the applicant—people are expected, after all, to value their family, friends, even careers—but this precaution has kept the RHC from ever having produced a traitor in the thirty years it has been active."
Basically, the Spells
Detect Thoughts and
Zone of Truth are a thing. And since you
willingly agree to undergo the test, that's
deliberately failing the save. Or having a really good Bluff/save/etc. Not really viable for a green recruit being interviewed by a high level/experienced officer who has been there, done that, and bought the T-shirt too. The fourth Yerasol war was not that long ago, and some of the staff are ex-military just looking for an excuse...
If they aren't legit, don't hire them. If they actively harbour a grudge against something that is directly opposed to the RHC, arrest them. You don't even have to have it "in front of" the PCs, just have it in the paper a week before they undergo the test...
e.g.: a person trying to join has been found to have - in their University/Docker/etc days - done a couple of borderline militant/anarchist things... (basically not your normal grievances, some thing major...)
They've since been more publicly genteel about their views but still harbour some secret resentment (for insert background, blame RHC/Risur/King, blah blah...), and although they aren't currently actively thinking about blowing stuff up/smuggling/
themselves, they know about a couple of people who might be trouble in future and are willing to let them dirty their hands for them... But they thought they were strong-willed enough to get through the "mental exam" so they could get their hands on some exclusive info that would help their "friends"...
This isn't a barbaric culture, it is (mostly) civilised... although execution, banishment and long prison sentences are not off the table; more details on this are also in the same section of the player's guide.