In my observations, someone who say he/she is not lying on a particular (but important) point, usually know he/she is believed to have been a liar before.
Romans 9:1 "I say the truth in Christ, I lie not, my conscience also bearing me witness in
the Holy Ghost,"
2 Corinthians 11:31 "The God and Father of our Lord Jesus Christ, which is blessed for evermore,
knoweth that I lie not."
Galatians 1:20 "Now the things which I write unto you, behold, before God, I lie not."