That's overly harsh. No, he can't individually interview every single person. But he can use whatever trusted supporters he has (and from our later talk, there were a good few using this as a way to get out of sylvania, and they'd be experienced in sussing out motivation), and he can judge groups as a whole and by their leaders (a mercenery band vs settlers, people known for discipline vs a group that made trouble before, etc), plus any notably problematic people (gossip and such).
If he's got 10 subordinates (he's probably got more) and groupsize is about 100 (which may be too large, but not by that much), then each man would only have to work through 10 groups. That's very much doable. You can fudge that number up and down, but it stays in the range where the journey would've given him enough time to get a decent grip on things.
Some good man in groups judged problematic may have died, and some bad men were not noticed (Kragg's dwarfening complicates things further, but not even Kragg knew he'd be doing that). But we have no reason to assume incompetence on his part, and some reasons that show he knows his stuff, so the assumption should be that he had could cause to believe there was a danger (the thread agreed), and that his actions were at least somewhat effective towards that goal. I don't think there's any indication he deliberately increased casualties, just who'd receive them first.