Before playing online matches, I went into practice and tested each gun and it's respective spray pattern. It's also useful to practice bullet effects on the bots in practice; their model sizes match agents exactly. I found this helped tremendously. The more time you put into practice and learning each gun, the more accurate you'll be.
If Riot ever releases a replay mode, it'll be much easier for players to recognize what they believed to be a headshot as a whiff.
