Llanfairpg, very good question.
I'm not an examiner, but from an instructor's point of view, different students need different techniques. For example, one recent student of mine, when I was briefing him or debriefing him, would always nod his head, agree with me, and often finish my sentences. It seemed, at first, that he was taking in everything I was saying..... but I very soon realised that he was actually understanding very little of it, and the nodding and finishing sentences was nothing more than a habit.
My technique with this particular student was to ask lots and lots of questions - far more than I would with most students. If there was something that he wasn't getting, I would ask him the question, then, when he couldn't answer it, I would tell him the answer, and then ask him the same question worded differently. Even doing this, I often had to repeat it half a dozen times before he eventually clicked that he knew the answer (because I'd just given it to him), but it was the only way I could find of judging when he actually understood something.
As an instructor, I get to spend lots of time with students, getting to know them and their habits. As an examiner, you don't have that luxury, which I would imagine must make it a much tougher job for you than it is for us?
FFF
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PS Just realised I haven't actually answered your question at all, but hopefully have added fuel to the debate!