The westerner who is able to speak, read and write Thai is not, in my experience, a particular rarity.
Calling someone a liar, in any language, is not a good basis for mutual understanding. It is, however, sometimes justified. For example, when Bob Dylan responded to the shout of 'Judas' on his tour of the UK in the late sixties.
If you think someone is lying you can always tell them that you find it difficult to believe what you are being told.
It all depends wheher you are looking for insult (and its possible repercussions) or understanding.
I always prefer understanding.