I would just add that insight in my games is not an automatic lie detector. The PC may pick up on the NPC being nervous, arrogant, dismissive. They might get a sense that the NPC is not being completely honest. For example, when people lie they tend to be overly specific and have well-rehearsed answers.
On the other hand I don't see anywhere in the rules where it says anything about magically knowing bonds, flaws, traits and ideals. I have no idea how you would gather that info with out mind reading.
Yeah, I get that. I've tried that approach but I struggle to do it well. I don't know how to fine-tune my roleplaying so that there's any ambiguity: if I display any kind of nervousness or whatever, my players immediately conclude the the NPC is lying. So why not just let them roll and tell them the result?
The underlying problem is two-fold, and applies equally to trap detection:
1. There are no simple mechanics that combine positives, negatives, false-positives, and false-negatives, especially without secret rolls. So there's no mechanical determination for when I should telegraph nervousness
even when the NPC is truthful.
2. And even if there were, lie-detection shouldn't really be a binary kind of thing (at least, without magic). Starting from an assumed 50% chance to randomly guess right, "success" should increase that chance, but not to 100%. I definitely can't roleplay well enough to telegraph
that.
Given all that, I don't try. Anyway, I think it's much more exciting when the players, through their own creativity, uncover evidence that the NPC is lying or not. It might even be proof, not just evidence, but mere evidence more accurately produces the kind of uncertainty that I think makes the game more interesting. "Ok, we're pretty darned sure he's not lying...but if we're wrong and we let him go we're goat$%@#ed."