Labour are going to introduce lie detectors in to benefit officer in order to catch out those people who are lying when making claims.
I wonder what response a lie detector would have given if Tony Blair had been using one when he gave the "dodgy dossier" details and when he misled parliament over the 45 minute claim.
The problem is that I think Blair believes his own bullshit so even a lie detector couldn't catch him out even when he is lying.
So in answer to the question, how can you tell when the Prime Minister is lying ? His lips move. Yes, its an old one, I apologise.