Keeping in mind that lying is stressful, what externally detectable signs might a lie detector identify, assuming it reliability

Lie detection or polygraph testing relies on Autonomic System arousal.  For polygraph lie detection, scientific validity rests on the strength of evidence supporting all the inferential links between deception and the test results. It is appropriate for making the discrimination between deception and truthfulness. Polygraph practice is built on comparing physiological responses to questions that are considered relevant to the investigation at hand, which evoke a lie from someone who is being deceptive. 

fir trees are found in areas with cooler climates. fir trees have needles for leaves and remain green year-round.