You ask him a question.

He answers you.

You know he's full of it.

"I swear to God!"

Here's how to get him to cough up the real truth.

-Nothing too nefarious required here; either tickle him until he turns blue and finally admits he was lying, or refuse to feed him. Eventually he'll get really hungry and crabby and just tell you the damn truth already

-"I promise I'm not lying"

-"Oh, okay. I see."

-(Turn to address Alluka) "So what do you have to say about this, Nanika? Is Big Brother telling the truth, or what?"

-Make a mental note of whatever whopper he just told you, then wait a few days

-Ask him casually about the same subject, just to see if he can keep his stories straight

-Oh, fuck. This is Kurapika we're talking about...of course he can keep his stories straight...you're shit out of luck

-"That's a fact, Jack"

-"You sure? I think you're lying"

-"Me?!!!"

-(Stare at him impassively and don't take your eyes off his face for several minutes)

-"Okay, okay, fuck! Fine. I lied. Just stop it already!"

-You are so used to him being a baldfaced shameless liar that you just go ahead and assume the opposite of whatever he says is the actual truth, so this scenario doesn't require much more than a minor bit of mental gymnastics on your part

-Before he has time to stop you, yank one of the needles out of his vest, and jam that fucker right into his forehead. That'll do it. But move fast, he's got good reflexes

-"Of course I'm not lying. Here," (he looks around and finds a Bible) "I'll swear on this. The whole truth and nothing but the truth, so help me God"

-"Bullshit!" you say. "Bibles are for kindergarteners! Go get that weird-ass book you're always toting around and waving in people's faces, let's see you swear on that"

-Yeah. He turns sort of pale at that one.

-Tie him to a chair, stick a sack over his head, and fire up a power tool so he can hear the menacing buzzing sound

-Boo can dish it, but he can't take it

-You really don't think Shalnark has it in him to lie about anything, so you just go ahead and take the boy at his word