Also as to Rojo's big relativistic point, yes your bias will distort your perception BUT regardless of someone's perception, there is an underlying truthful answer to the question.
"Is he a liar on not?"
Would you care to re-phrase your question?
EVERY SINGLE HUMAN BEING ABOVE THE AGE OF 2 YEARS OLD IS A LIAR (probably most who are under 2 are as well, though it is a bit harder to pin them down). As a result, your question, "Is he a liar or not?" is equivalent to asking, "Is he a member of a set of beings WHICH CONTAINS THE ENTIRE ADULT HUMAN POPULATION as a subset?"
Again, care to re-phrase?