To be clear then, you are saying that if I (the Rogue Inquisitive character's player) say to you (as DM) "My character suspects she is being lied to. I want to make a Wisdom (Insight) to see if the Barbarian character is deceiving her. My character has Ear for Deceit and Expertise in the skill so she can't get less than a 15 here." As DM you'll allow the Contest of skills to be made and if my result is higher, then you will tell me if the Barbarian character is (or is not, as the case may be) lying?
Personally I would just say that the speaker seems sincere. As far as you can tell they're telling the truth as they know it. Insight is not Zone of Truth any more than persuasion is Charm Person.