I honestly think we're pretty terrible at that, including me. We (well other than a couple people who have kindly not gloated TOO much about it) didn't clock him as an habitual liar. At least not at first. And not after a little while. And not after a lot of while.
It took several years of everything he said, and every time it got checked, and every time it got checked turning out "hey wait this guy lied" before it finally added up to "wait wtf man every time this dude says anything it is a fucking lie" before we reached that conclusion. To be fair, sometimes lies weren't coming out of his mouth. It also took us a while to figure out that that was because cocks were going into it in the times lies weren't coming out.
Let's just say at least this thread was really bad at detecting pathological liars and it took a LONG time for us to figure it out.
Of all the insulting names we may actually deserve, perhaps we should be called the Pollyanna Farms for our trusting nature and optimistic outlook on people's motives.