Saying let me know, isn't admitting or accusing themselves. I think they are trying to pin point which mannerisms you think relates to them on drugs or alcohol.

Im going out on a limb here, I doubt that they are under the influence. Perhaps you think they under the influence because they arent responding to how you'd like in some way. And trying to find a reason.

Sometimes I come off as insulting or blunt. I understand it is in my matter of fact way of speaking. But I try not to accuse people of things, that opens up a can of worms.