I understand what you are getting at. Sometimes, in doing "flagged target tests", you can ask a person "Do you hear this ?". They will scan it, and announce "yes". Simply because they heard a beep. But as you know, that's not the full story. The REAL story is: Would you have known this target vs another target. Ie.: pass or dig, etc....
There was a fellow in CA with a new-introduction machine, that wanted to put-it-through the paces, on known targets. So we agreed to meet at a certain turf zone, where I was fairly certain that I could still flag suspected deepies (wheaties/silver), for him to try. And ... each time, he'd proudly announce that he can hear it, and would agree with my consensus of "deep wheatie or silver". Heck, he could even pull out the headphone jack, and ... sure enough, there'd be a beep of sorts. Hmmm.
But after awhile, I began to notice that he wasn't getting any deep suspects for me to sample. And I began to suspect that there wasn't subconscious bias at play, since ...: When someone points out a signals and says "deep silver", and you know this person is generally correct at this location, then the subconscious reaction is to AGREE with them. And interpret ANYTHING you hear as being just-as-the-person is telling you.
So I played a trick on him. I purposefully flagged something I thought was lousy . Like a nail false or whatever. And then ... when asking him what he could hear, I lied and told him it sounded like deep silver. He waved over it, and gleefully agreed with the assessment, and that his new machine agreed, etc.... Then I revealed that I had been purposefully deceptive, to see if he was being tricked by subconscious biases. When he realized what I'd done, he changed his tune, and said "come to think of it, it *did* sound kind of junky". Doh!
But with 2 long-time skilled users, who are aware of the mental mind tricks, I think that flagged target tests CAN be done.