To call one out on a lie, generally when someone says something so incredibly unbelievable that you're sure they must be lying. Based largely on the old centure honor system, the accused will usually admit to a lie after being called out on a bluff check.
Keith: No, no, no, it gets better... then I pull her wedding dress straight out the closet and take a fat piss on it!
Jon: Woah woah woah, bluff check man.
Keith: No lie doggy, swear to god!
Noah: Its true man, I was there.