How do people know that they were reported after a match?

I played a few matches today and I would mute toxic players during the match (I've been more in line with the tribunal/ summoners code after my ban 2 months ago) after the match in the post game lobby, right after I reported a player they would say things like "wow someone reported me for X" whatever it was. I know about the warning system (although it doesn't always work the way it should) I know if you are reported a lot for the same thing you get a warning message after you leave the post game lobby. But these players clearly know what they were reported for during the post game lobby immediately after a report. Does anyone have any idea's how they know what they were reported for? because if so I would love to be able to know how, I feel like knowing my team thought I was too negative immediately after a match would help me immediately become more friendly to my teams.