In the Radical's Handbook there is a power Zone of Truth. The target finds it very hard to lie when under its effects and cannot lie if he fails a Willpower test (great for interrogations). However, he can opt not to answer the question, it says.

So does the target somehow know he cannot lie?And thus may choose not to answer the question because he knows if he does he will tell the truth? (ofcourse, a target not answering is pretty much incriminating himself).

How does one know that one cannot lie?

The Laughing God said:

I would venture that they may not notice right away, until they fail the WP test and try to lie and either A) they find they literally cannot speak or B) say the truth even when they did not mean to. They might connect that and what the psyker is doing, but they may not. I would just play it like that though.

A more interestng question is: Can the psyker know if the target succeeds the WP or not? If so it is a sure way of always getting the truth.

Choosing not to answer is seldom possible when interrogated by the Inquisition, unless the target is Fearless and/or immune to pain.

The power makes it difficult to tell a lie, and as such I would rule that subjects under this effect would notice the difficulty. They must pass a Willpower test to overcome this difficulty, or if they do not, they cannot lie. It doesn't say they MUST tell the truth, so I don't think they would intend to lie, fail the WP test, then tell the truth.

And I also doubt the Psyker would be able to intuit in any way whether the character had passed or failed their WP roll. This could cause inadvertent confusion in most cases, but in the situation of interrogating an EXTREMELY intelligent individual, their prisoner may find some way to turn the tables on them by providing them with false information under the guise of not being able to lie.

Ultimately, investigation is such a critical part of the game that Zone of Truth should expedite and simplify interrogations, not trivialize them altogether.

The power makes it difficult to tell a lie, and as such I would rule that subjects under this effect would notice the difficulty. They must pass a Willpower test to overcome this difficulty, or if they do not, they cannot lie. It doesn't say they MUST tell the truth, so I don't think they would intend to lie, fail the WP test, then tell the truth.

And I also doubt the Psyker would be able to intuit in any way whether the character had passed or failed their WP roll. This could cause inadvertent confusion in most cases, but in the situation of interrogating an EXTREMELY intelligent individual, their prisoner may find some way to turn the tables on them by providing them with false information under the guise of not being able to lie.

Ultimately, investigation is such a critical part of the game that Zone of Truth should expedite and simplify interrogations, not trivialize them altogether.

For no particularly valid reason, when I imagine someone unable to lie I hark back to the Sanctioning effect of stuttering when speaking of deamons.

Inquisitor - "did you commit [heresy A]?"
heretic - "I did nnnn... naaa..nuuuh"
Inquisitor - "you did? well then" *blam*

Of course a more savvy heretic would be less obvious so perhaps a decieve vs scrutiny test is in order to see if the heretic can conceal whether he passed the willpower test or not.