When an experimenter want to find out about e.g. what is the hypothesis perceived by the subjects via post-experimental inquiry, if the subjects answered that they did not perceive any hypothesis, it might be helpful to find out if the subjects are speaking the truth or lying out of good intention (so that the experimenter would not know that he/she has wasted time on a potentially useless data point) with lie detecting technique.
Aside from the effectiveness of lie detecting technique, is it allowed by e.g. ethical board?