I don’t know whether I got you right: Your question is how a NPC, while actually telling the truth, can convince the players that he is telling the truth?
There are two solutions that come to my mind:
- You could ask the interrogating player to make a check; the NPC would actually assist because they’re telling the truth. I would set a rather high difficulty for this, representing the distrust of the interrogating character that has to be overcome. You could also make an extended task, especially if the characters need the information of the NPC within a certain time.
- You could simply rule that since the NPC is actually telling the truth, they do not need to roll at all and it’s just about the players and their judging of character, so they need to role (maybe Insight + Command or Insight + Security?). I would set the difficulty to 1 or 2 as basis, maybe raised for certain species (i.e. if the character is not familiar with this species, it would be harder for them to ‘read’ their body language etc.), you can further modify this by spending threat.
I hope this helps.