iserith
Magic Wordsmith
If a player says. ‘Does the guard believe what we said? Can I use insight to see if I can tell?’ That is fine.
But if that same character has a big expertise bonus in insight, then that screws up the DM adjudication. For one thing it becomes more about exploiting the bonus, and less about narrative nuance. But mostly, the DM gets a feel for difficulty for what should be challenging and what should be easy. The big bonus expertise is playing a different kind of game.
It would not be fine at all at my table to say "Does the guard believe what we said? Can I use insight to see if I can tell?" regardless of the character's Insight bonus or whether he or she had Expertise in it. Because the player (1) didn't describe what the character was attempting to do and (2) asked to make an ability check which isn't the player's role in the game. It doesn't screw up the DM's adjudication in my view as there is nothing there to adjudicate.