I know where your coming from Vince, but I do take a hard stance on this and if I feel a "white lie" is on the cards I will always, politely, explain the fact that I know.
Its always a good game to play when a customer gives you a reason and you offer the solution back at them.
I realise you will not always get to the truth of the matter, but in general I believe we have a large percentage that do tell the truth.