I've had two especially notable experiences with pathological liars, discounting smaller encounters with hucksters and various sales/marketing types (not all, but some, and an absolute showstopper when it occurs).

The first was at a part-time job during my days at uni. A new guy had joined the crew, intelligent and affable, but it turned out, simply a pathological lair. We weren't aware of this until ... I forget precisely what had occurred, but apparently lying when filling out bank cheques was among his failings....

The second was a more personal relationship, in which it transpired that I'd been lied to from the very beginning. This person displayed many of the characteristics of borderline / narcissistic personality disorder, and would often become wildly (and increasingly inappropriately) upset under questioning, deflecting or projecting questions and accusations back. I'd had little experience with the behaviour previously and it was exceedingly disconcerting.

This essay on diversion tactics covers many of the behaviours I'd experienced in that incident (I'd submitted it to HN a few months back, though there was no traction):
https://thoughtcatalog.com/shahida-arabi/2016/06/20-diversio...

What ultimately broke that case for me was:

- Finding several people who'd known something of the person's background, and comparing notes. Inconsistencies immediately started popping up. Score one for the consistency rule of truth determination. Various audit methods rely heavily on these.

- Realising I had a trove of communications from the person (legitimately obtained --- they'd given them to me, though some reverse-engineering was required to recover data, both points that held significance in later proceedings). It took some patching together of these to generate a clear picture, which was hampered by the false / incomplete story I'd been told, but the full story eventually was clear.

Eventually we were able to pin down the individual under testimony showing the scope of deception and the fact that it was intentional, premeditated, and predated my encounter with the person (I happened to be the sufficiently clueless victim appearing at the right or wrong time, depending on whose viewpoint you take).

Many standard bullshit-detection tactics (see several referenced in https://old.reddit.com/r/dredmorbius/comments/28ge14/on_nons...) turn out to apply. Sloppy logic, imprecise language, deflecting questions, various forms of special pleading, turning to accusations against the questioner when the situation gets too heated, etc., vagueness as to facts, and inconsistencies within the story (these can require close listening) were ultimately the tells. Even once presented, there was a lot of work left to uncover the actual truth.

In more mundane dealings, what I find is that fairly sensitive and alert to unqualified claims, inconsistent actions, or statements at odds with facts of which I'm aware. It's interesting to see just how often this occurs (I'm thinking of examples from an appliance purchase, broadband service calls, and medial services, within the past year or three), and once those occur, I call the person on their bullshit. I've walked away from purchases or services strictly due to such behaviours --- if you cannot be trusted to tell the truth or admit to the limits of your knowledge or understanding on the simple stuff, much as with Van Halen's brown M&Ms, why the hell should I trust you with the complex bits I don't have knowledge of?

There's a book by an underappreciated author, Jeremy Cambbell, which I've yet to read though it's premise is fascinating. It's The Liar's Tale: A History of Falsehood (https://www.worldcat.org/title/liars-tale-a-history-of-false...), and concludes that lying is a universal behaviour.

(Campbell's other notable book, the 1982 title Grammatical Man, is about information theory, and was a fascinating introduction to the field for me.)