If such a machine exists then it's either a total ban on using it as evidence or the problem of "forced voluntary test": if you don't volunteer, the court infers you have something to hide. The govt doesn't even need the warrant in this case.
The only way (currently known) is measuring brainwaves.
This can only occur during activity, in witch a question has to be asked, and the the participant will most probably recall the memory.<p>As a result, this is avoiding laws (which is lame) and in a way you participated in incriminating yourself, but without your consent.<p>So it's torture without the pain.
There is a way that scientists have been able to analyze photos to discover the pulse of the person in it. Its very interesting, though I do not know the deeper details of it.<p>I am too lazy to find a link, if anyone else wants to share it that would be great.
Could the people require those-who-write-laws' thoughts be recorded first? Imagine how much better our government would be if we knew 100% why they do what they do.