You pair them randomly so as two co-conspirators can't arrange to work together. You make sure the ones who are paired don't trust each other. You tell them that they are expected to report any suspicious behavior -- like someone mentioning the constitution -- and that they will be tested. You do test them. They get paired with an incognito auditor whose job is to offhandedly mention that what they're doing is violating someone's rights. If they don't report him, they're disciplined.

Of course, all these things also make it hard to convince them that they're the good guys.