From the reference doc.
"Glib Lie (Su): A master spy of 2nd level or higher can deceive truth-detecting magic. A creature using this sort of magic against the spy must succeed on a caster level check against a DC of 15 + the master spy's class level to succeed (as if she were under the effect of a glibness spell); failure means the magic doesn't detect the spy's lies or force her to speak only the truth. This ability does not give the master spy the glibness spell's bonus on Bluff checks."
And Glibness:
"If a magical effect is used against you that would detect your lies or force you to speak the truth, the user of the effect must succeed on a caster level check (1d20 + caster level) against a DC of 15 + your caster level to succeed. Failure means the effect does not detect your lies or force you to speak only the truth."
The way I see it a caster of equal level will have little trouble defeating this ability. For example I am level 9 with 2 levels of master spy, making the DC 17. A level 9 caster only has to roll a 9 or higher. This seems pretty weak (and would get comparatively weaker when I have to take levels other than master spy at 18/19/20).
