It depends on the lie and on the person. I can tell sometimes - but I wouldn't say I can always tell if someone is lying. Most of the time I take people on their word I'll only get suspicious if the story just doesn't make sense or if they get defensive.
I agree with Alex, depends on the lie, depends on the person. To some, lying is a day to day thing. They are hard to outright catch because they do it so often, it's a matter of course. These kinds of people do eventually get caught though, you can only lie so long before everyone starts to know you as a liar. Other people are easy, it's like they are bad poker players, the tells are everywhere.