Lying is easy. You tell your lie, roll a Bluff check, and your opponent(s) roll a Sense Motive check. If they win, they know you were lying (or in some cases they think you were telling the truth but the action you wanted them to take was too much). If you win, they believe your lie and will take reasonable action in response to it.

What's not so easy is telling the truth. What happens when you tell the truth and someone doesn't believe you? What check is used in that case?

The Book of Vile Darkness, page 38 discusses Bluffing regarding torture, where the person being tortured may give false information in order to get the torture to end. However, again, there is no check to confirm when the truth is being told.

The only thing I've been able to find is a DC 20 Sense Motive check for 'Hunch' with which "you can get the feeling that someone is trustworthy."

Is there anything else or is always just a DC 20 Sense Motive for someone to believe the truth? That seems weird considering a lot of commoners will fail that check.