Unfortunately, liars can't be outed by their pants being on fire, though it would be great if that did happen. All those pants going up in flames... In real life, things are little bit different. Even the lie detector isn't a foolproof way to find out that somebody is lying. Ugh, what is a regular person to do to get out of this web of intrigue?
Well, there are tell-tale signs of someone lying that you can look for. So while you don't have one weird trick to tell a liar (like checking whether their pants are aflame or not), there are multiple tells that can combine to out them. Learn them good!
Liars don't mention themselves.
Liars avoid talking about themselves while they... lie. Speaking or writing about others, especially in third person, helps them put distance between themselves and the fib.
Liars will use negatives in speech because of the subconscious feelings of guilt about lying.
Liars tell their false stories in simple terms. Judgment or evaluation are hard to make, so the brain goes for simple lies every time.
Those simple explanations will be enhanced by intricate sentence structure and fancy words. There might even be plausible, small details inserted to amplify the deceit.
Bonus round: the eyes
The eyes are the mirror of the soul, even when you lie. Lying makes you nervous, leading to pupil dilation.