iserith
Magic Wordsmith
Since [MENTION=97077]iserith[/MENTION] said that he'd handle the same situation one way if the speaker was an NPC and another if the speaker is a PC, we've already proven there is uncertainty. There wouldn't need to be a check for an NPC if there wasn't.
I will refer you back to the post you're referencing with some additional emphasis:
iserith said:If a player describes the character as trying to determine an NPC's true intentions by observing its body language, speech habits, and changes in mannerisms, then I'll adjudicate and narrate as normal. I would have telegraphed a tell while describing the environment anyway.
In almost all cases, if there is an ability check at all, this would be a Wisdom (Insight) check though; passive Insight would only be called upon to resolve a task performed repeatedly and that's not likely to come up much.
I was very careful with my language here. "Adjudicate and narrate as normal" means to follow the D&D 5e rules for adjudicating a proposed task which does not necessarily mean there is an ability check. I go on to clarify that there may or may not be an ability check.