Magic

    What's the best way to ask someone a question and get answer? When they don't know it's happening.

   How do you do that?  There's more than one way but the problem is with reliability.

   "Truth serum" will loosen some people's tongues.  But not everyone is comfortable with needles with or without up front notice.

   Some people talk or mumble in their sleep.  But to guarantee anything said is response to a question is impossible.  You need to have the person conscious or there's no value to the answer.

   That leaves subliminal techniques on the table.  If you have a way to pose a question that avoids having to deal with a person's conscious mind you're all set. But you still run the risk of them becoming aware of it and getting a little upset.

   You'll need to be a little more circumspect than that.  In fact, you'll need to apply subliminal techniques to get them to say that they said the answer (whether they actually did or not) which closes any loopholes.

   So how do you pull that bit of magic off?

I'd vote for constant and continual harassment of a psychological nature.  Given enough time to wear someone down you could tell them to say anything.  Quite a few of them eventually would.  Most people aren't willing to live that way forever.  At some point they break.  Examples of phrases to use at that point:

• Do you know what you just agreed to?
• Let me get this straight...
• We got 'em!

   At this point you'd think power lines would be in the picture. But that's silly.  Someone might not always be near a power line.  They aren't always around us and they can be turned off.

   It's probably a good thing that infrasound and ultrasound can be used in place of power lines. Once everything's up and running then someone in a car, or business, or at home has no chance.