Note also that Voldemort himself has some definite abilities in this area: "He lies! He lies!" and "Do not lie to Lord Voldemort. He always knows." So that might support firelocks's theory.

Also, Voldemort's built-in lie-detector feature may add to the difficult of Snape's next mission; even if he is disguised as Crouch Jr., I would expect some clever evasiveness to be necessary from time to time. (In my fanfic, Snape managed to block Voldemort's mind-reading with a strong surge of "hatred for all things Potter." Whether this would work in the real story, I have no idea.)