Sure, but the OP is talking about a person asking questions about what the enemy did. And that question was answered in this exchange. Because the paladin said they were spending a 3rd level spell slot on Divine Smite. That is the technical information that was being asked about in the OP. Same as the player saying they are using colossus slayer, or hunter's mark or sneak attack. The DM also never needs to ask "is he a rogue" because you should know if one of your players is playing a rogue.
Let's look back at the questions the OP gives us.
Exchange #1: "Wait, how is that 6 points? Why did you roll another die? Is he a rogue? <PC-1> isn't flanked, so there shouldn't be sneak attack damage."
There seem to be three questions here, but the OP lists them all together. P5 in this instance isn't waiting for answers, he's thinking out loud. Q: how is that 6 points. A: Oh wait, I see another die. Q: Why did you roll another die? Observation: Must be some sort of ability. Q: (Possible answer) Is he a rogue? Complication: P1 isn't flanked, so that doesn't work.
The player, to my mind, is clearly thinking out loud, and following the logic. He isn't disputing that the character took 6 damage, and he knows it must be some sort of ability of the bandit, but the ability he can think of doesn't make sense in the context. And, this might be important. Was the bandit using magical arrows? Do you take more damage if you are standing in the light? Is it poison? These all matter, and some of them could be the DM hinting at something else. Then the OP confirms that is wasn't sneak attack, and there are more questions.
Exchange #2: "It's all piercing damage? So it's not an elemental buff. Is he a Ranger? Oh, <PC-1> was already wounded, is it extra damage from Colossus Slayer? Isn't that a d8? Wait, did you roll a d6 or a d8?"
Again, this is mostly thinking out loud to my eye. Q1: It's all piercing damage? Conclusion: So it's not an elemental buff (this eliminates many cantrips and spells like elemental weapon.) Q2: Is he a Ranger? (one of the only other classes that reliably adds non-elemental damage to their attacks without a save) Observations: Oh, P1 was already wounded Addendum, assuming enemy is Ranger: is it extra damage from Colossus Slayer? Complication: Isn't that a d8? Q for Clarity: Wait, did you roll a d6 or a d8?"
I'd say it is important to note that in both strings, the last part is the only real question.