You have given only a very limited amount of information and I wonder if the key sentence is really "...how far do you take it when you want to figure out if they are telling the truth?" Does this mean that you already have misgivings about the "truth" that has been presented to you? IF so, are you seeking support for a type of due diligence on the person i.e as in a google search...acting as investigator... check other records...etc,?
There are two different interpretations and concepts expressed by others here in this forum, so I am just wondering what the direction of your question actually is.

With my life, I laugh to keep from crying.

So most who know me, know better than to take me seriously about 3/4 of the time.

One thing im not is mean. I will give you a chitload of crap, but its all in fun.

And yes, I laugh at myself alot!

It's what people don't say that tell us the story. It's "how" they say it that speaks the truth. Actions speak louder than words. Always, always....

See what I mean? lol

Just a general question, if I want to know the truth I know how to figure it out...but now we are saying that at some point I'm calling someone I hardly know a liar, which isn't fair to anyone. Even if someone were to lie to me it isn't like I'm going to use my available resources to search out the lie, as soon as someone lies to me and I catch it without a doubt the relationship is over...I don't have time to chase a ghost story.

People who lie should be tarred and feathered. And then covered with honey and left on an ant hill. And then dropped off in WV to live out the rest of their days. That will teach 'em.....

This is a good one, because there are several character types who exist and they are almost verbatim of each other and there are also other ones, who seems to be more independent, but never less still become suspicious after a while.
Sounds weird, but after a lot of talking and chatting many gain the experience to smell the BS not right at the moment, but let say...maybe a few hour later, after going into a more serious thinking and thinking over the conversation and try to create a character in our mind of this person. We do have the capability of drawing out a person in our mind, imagining this person, and when it looks all screwed up and major parts missing, the lies become very visible.

I don't take it too far. If I feel like someone is being less than honest and it's more than just some little white lie deal, then I split. I don't need cold hard proof at this stage of the game. If something even appears to be amiss, that's good enough for me. If it's something concrete I might ask them about it and see what kind of response I get, otherwise I just gracefully bow out.

I had an ex that used to think good intentions were the same as truth. I disagree with this. If you can't back up what you say with action, then you're basically just doing a lot of talking (lying). You only disappoint people when you talk a good story but it never comes to fruition, no matter what your intentions were.

Unless it's someone I've dealt with in the forums or through e-mail, I generally invoke the "Law of Opposites" when evaluating someone on line.

If she says she's "unique" (and what's the point in saying that? -- it's the sort of thing that has to be demonstrated), she isn't. EVERYBODY says they're "unique" in their profile, and all of the profiles are the same. It's a bit inconsistent.

If she says "looks don't matter," they will. If she says "I like sports," she can't tell a baseball from a yacht. But people don't write the truth in profiles -- they write whatever they think they NEED to write in order to come across as being "acceptable."

Unfortunately, whatever they think they need to say is exactly the same thing the last 48,882 people have also felt they needed to say.

"Proof," to me, is displayed through consistent behavior/expression in the forums, etc. I refuse to take anyone on line automatically at "face value" -- I've been through too many arsonists, phony bank tellers, lazy newspaper columnists, phony kidnap victims, etc., for that.

Simplicity in language and message, I've found to be telling signs of wisdom.