Trust/Belief

[Snowmantheory]

Recently my group has come across a problem of party members believing each other. Someone will say something totally unbelievable and we have to figure out whether or not we trust him. At one point we were just deciding if our characters trusted the other guy. Then my DM incorporate a system, which we all agree doesn't make sense, that we're trying to work out. The current system is the person trying to convince us of the truth rolls a diplomacy check combatted by our sense motive. If we beat them, we don't believe them. That doesn't make sense because a high sense motive check should tell you the truth about what they say, not allow you to be stubborn and disbelieving. I then thought that maybe you don't believe them if your sense motive is lower than their diplomacy. However, that means the more convincing they are, the less likely you are to believe them. Has anyone else run into a similar mechanics issue? Is there a good system for this or should it be based off of our characters and role-playing? (To clarify, this isn't a bluff check. This is a system for if someone is telling an unbelievable truth.) Thanks!

[Honest Tiefling]

Typically, social skills like bluff are not used against the player characters, as it often makes players feel like they are at the mercy of bluffmonkies. I would say, just go off of role-playing and characters. Try to work it into the story, and work with your own characters to keep things going. A stubborn unbelieving character might go along with the town crazy just to see them be proved wrong, for instance. They still don't believe, but they have a reason to keep going with the plot and stop arguing once it no longer becomes fun.