Ah. Yes, indeed the players could decide it’s false unprompted. In my view, that’s a decision about what the character thinks, which should be left up to the players, not the dice. However, having decided that they suspect it’s false, the players could of course decide to do something to try and confirm that - perhaps examining the NPC’s body language and mannerisms to try and discern clues about his intentions. At that point, it might be appropriate to call for a check to resolve the action - I think Wisdom (Insight) contested by Charisma (Deception) would be particularly appropriate. If the player succeeds, they would accomplish their goal of detecting some indication of the NPC’s intentions in their body language, which I would then describe. If they failed, they would not succeed in their goal so I would not describe any such clue, and there would be some consequence for failure - perhaps the NPC notices that the PCs are suspicious of him, and becomes hostile. Whatever the consequence may be, I think it’s best practice for the DM to inform the players of that potential consequence before they commit to making the roll.