There's actually a WoJ that even the WC doesn't have a surefire way to detect lawbreakers. Soulgazes help, but only in cases where the black magic is so extreme you probably don't need it anyway.
OK, thanks! Can you point me to that WoJ?
I recall the execution from the beginning of
Proven Guilty, where soulgaze was mentioned as a key bit of evidence. Granted, this was an instance of "black magic is so extreme you probably don't need it anyway." But if it's not really needed... why bother? Who wants to muck around in such a black soul??? You Denarians over in the corner, don't bother answering! Later in
PG, Harry 'gaze's Molly, and sees Warlock-Molly as a possible future, so the potential IS visible... I can see that it may not be "surefire" -- particularly if it's a senior wizard, who might have the discipline to hide stuff from the person 'gazing them -- but it is (at the least) "very useful."
My point on this is that the WCouncil -- and "good" wizards in general -- seem to have quite a panoply of "Dark Detectors" available to them. Soulgaze, Mai's stone guard-dogs, genuine Temple dogs... in addition to what we've seen, we should probably presume other methods, which are off-stage (either not-yet-invented by Mr. Butcher, or intentionally not used) so far. The sum of all these methods would seem to make it incredibly-unlikely for a Lawbreaker to hide for long (if they don't get Soulgazed, they get sniffed by a Stone Dog, or caught some other way) had led me to believe that there
must be some way for Lawbreakers to intentionally hide their status (at least from other Wizards).