How do you deal with a person who, you know, is not telling you the truth?

Best way to handle a consistent liar - separate yourself completely from them. Clearly if they feel the need to be lying all of the time, then they have some personal issues of their own. Why would anyone want to be with someone they couldn't trust.

If you cannot separate yourself from this person. Call them out on it, and frequently. Let them know that you won't stand for this constant lying and there is no reason that you should.

Usually there are other ways to find out the truth than through that one person.....
usually when I've identified a person that lies to me consistently
I'll get fed up with it one day as they feed me lies I back them into the corner and basically weave them around in circles until they can't be consistent with their lies......I'm pre-law so a part of me likes making them admit their own lies but it is a rather cruel and dirty method.
so if I'm not cross examining someone I'll smile and nod at the lies and take everything they say with the grain of salt and discover the truth for myself.....

Depends on who it is and on how much you care about this person. You can be amused by it, use it as entertainment. You can just take whatever they say with a grain of salt, or you can stop dealing with them all together. Its aggravating to say the least, I wish you luck.
What is it you want them to tell you anyways?

As a person who has a tendency to habitually lie, There are definitely things that people do to me to call me out.

When a little lie gets started, most people let it slide, and accept it, but if you know that someone is lying to you, Press on it.
If for example someone mentions some feat that they preformed, and you know that they lie, then interrogate them on the topic. You don't need to be snide about it, but show genuine interest in it, and you can make them feel miserable.

Often interjecting questions like "when" and "where" will yield similar results, because they put the lie into a direct context. If you get a really hazy answser for the "when" question, then the liar is running out of ammo.

I say, call out their lie, make them elaborate on it, and press them for the truth.

It isn't a good thing to lie, but some people are just too insecure to admit to a lie, or don't have enough self respect to tell the truth.

Often times passively exposing a lie will have tremendous results. Get them to admit that they added some information, or made something up, and the other lies will melt like Ice in your hands.

Asking little minute details about things and then asking them again later on and watching how they change their story. I find it pretty funny actually how they trip themselves up.

No matter what you do, you can't pull the truth out of a consistent liar. There are two possibilities that I see.

1) If the person can't seem to trust you for things you have done, or have trust issues based on other life experiences, then they will lie to you every time. There's nothing you can do to change this in them, except for allow them to trust you, Give them every opportunity. If they never trust you, then at the end of the day you can say that there was no fault in you.

2) Some people lie just for personal gain. They do it every time because of a personal agenda. Either they want to hide things that are personal, or they see some sort of benefit in lying. Situations like this won't be fixed until the person experiences the negative effects to lying. Some people just choose the hard way.

All you can do in either case is be trustworthy to them, and take everything they say for truth, yet find out always to be sure. Maybe when they're caught, they'll experience negative effects (loss of trust, disappointments. Etc. ) They may keep lying, but you can always walk away knowing that you were the better person.

Most of all, when you do actually catch them in a lie. . . EXPOSE IT! It might be painful to them, but unless they're found out about, they'll never learn.