As long as we're talking about circumventing the Dark Mark- The Mark turns visible based on whether the DE knows he's made, so they could bluff the Mark by arresting someone and acting completely convinced he's a DE before examining his arm, rinse and repeat.

I'm feeling thick at the moment - can anyone explain this passage?

'His gaze grew more distant. "Oh," Severus breathed, "he was very cunning indeed..."'

Perhaps he's just reflecting on how ingenious Voldemort's method of concealing the Mark was, but he's had almost twenty years to reflect on that privately. It seems sounds to me that Voldemort included some sort of charm that forced the Death Eaters to divulge a fake explanation of the method whenever it seemed like someone was getting too close to the truth in order to keep people from digging any further. That explanation fits with Quirrel playing "one level higher than you", but I am wary of looking for cunning plots hinted by every full stop in the text. It just seems like a bit of a strange thing for Snape to do at this point.

They didn't now that until just now, did they?