Often we know that someone cheats/lies, but we do not bother about it when nothing is at stake. And those fools think that you did not know that they are cheating. But in the end, this makes them cheat more often and also you start loosing confidence in them.
And at times, we even advice our friends, saying why fight with the liar, when nothing is at stake. But I think, if it is worth lying, it is worth fighting against. It is worth fighting just to say that you are not stupid. So next time you find someone lying, do let them know that you are not dumb.
BTW does professional life demands lying. I would say unless you are on such a profession as a lawyer, it does not. And the benefit from lying is nothing when compared to the ill-effects of being caught lying. Don't lie, and don't let anyone lie.