LMAO!!!! I see you had to deal with Mischief Maker, I am sorry you had to be put through that
Anyhow, on topic:
I think Kakashi would have noticed the sudden jump in chakra strain from awakening the MS which caused him to pass out.
So he either KNEW he had it or at least knew that SOMETHING had happened.
From that point on we don't know what he did about it. For all we know he could have attempted to work on it and train for it. Most likely he decided that it was too much of a risk given his chakra levels (especially as at his age in the latest manga).
Then, as you said, after the Itachi incident, he decided to work on it even more (during the timeskip) so that he was able to use it SPARINGLY. However, since then, as evidenced in the Obito fight, he has gotten better at it.
So I agree with you - I think he knew he had it - maybe not right at that second, but most likely soon after when he investigated the cause for the sudden chakra strain. (Kakashi is not an idiot )
That said, it IS still an assumption since it hasn't been shown outright in the manga, but logically it makes sense considering what we HAVE seen. So + rep for you efforts!
Side topic: Sasuke -> He awoke the 1-tomoe vs Itachi in the flashback. He awoke the 2-tomoe in his right eye in his fight vs Haku.
thanks man and lol yh mischief was really annoying , he disagrees yet cannot prove his disagreement and like u have said its is an assumption on my part but i show why think that way