But, in this situation, for those who really want to play it this way, the outcome should be the result of the player, not the NPC. The DC can be the NPCs roll, and instead of a roll (if you don't want to tip off the player) you could use passive Insight. And if you further don't want to tip off the player, use the NPC's passive Deception. If the player succeeds, narrate in a way that leaves a clue.
It's just like a trap, really: you don't "roll for the trap". You might use passive perception, or you might just let the players decide if they want to search.
Although, as I said up-thread, I think it's really hard to do clues without giving the whole thing away. If you add, "and he doesn't make eye contact with you while he's saying it" the player 100% knows you mean the NPC is lying. Which is why I wouldn't bother with any of this.
And again comparing this to traps:: if you sprinkle traps around randomly without broadcasting them, and the players are used to that, they will feel the need to "search for traps" all the time. And if that's the case, I can see why asking them to describe what they are doing (in searching for non-existent traps) instead of just saying "Can I roll to detect traps?" would be a pain. But if they know you won't leave traps without broadcasting them, and the broadcast gives them specific ideas about how to search and what to do, then they will try to describe actions that don't use dice. And, for many of us, this is a lot more fun than saying, "I'll roll Detect Traps" at every door, chest, desk, hallway, etc.
Similarly, when preparing an adventure, if there's a Big Lie as part of the plot, make sure to leave a clue. Perhaps prepare ahead of time a mistake the NPC will make in telling the lie (getting a detail wrong, one that the players should know, for example).
