Sure. Of course I could be wrong. If I want to get an idea of their emotional state I can also ask for an insight check. Then when I ask for that insight check, much like attacking an opponent, the outcome is uncertain so I roll a check.Can you, as a player at your table, just say that your PC doesn't believe the NPC?
There's no difference between swinging a sword and using a skill in my game. In both cases you can add flavor, but it doesn't change anything.