the boy who cried wolf

[Deaxsa]

help me out with this scenario. a man is known to be a liar, and the party encounters him a couple times, and he proves the rumors about him to be true (that he lies incessantly). However, things start to go to the dogs where the party is at, and in the hope of saving his life, the man goes to the party and asks for their help, however, he also provides a crucial piece of evidence that allows them all to escape easily if they work together. the man, in this case, is actually telling the truth.

here is my question: what do i (the DM) do to make it unclear whether he is lying or not? do i make them roll sense motive, and if so, against what? his diplomacy check? an involuntary bluff check of his, modified by his diplomacy to lower the bluff check?

[Vaz]

You make them do nothing. They do it on their own.

If they think he's lying, they can either attempt to discover he is lying by rolling a Sense Motive, assume the truth, or assume a lie. As he's telling the truth, then any test result would come back "you are unable to determine if he is lying", and then it comes down to whether the party believe him or not.

[Sylthia]

If you want them to believe the guy, you could have him produce some evidence that he is being honest. He could still be lying, but without that it sounds like the party would be foolish to believe him.