Premades should be visible to both teams.
Honestly, I don't know why this isn't already a thing (my best guess is that Riot can't find a way to show it aesthetically on the loading screen or in game without cluttering things up).
Toxicity usually only applies the opposite way (ganging up on solo/other players) -- not saying all premades are toxic, of course, but groups are much more likely to take a friend's side if flaming starts.
I'd really like knowing if i'm the odd man out. Not just for verbal exchanges, but it changes the way I play if I know my teammates are on voice chat and can coordinate plays more effectively than through pinging alone. Or if you are solo, and you're playing against a premade duo bot.
League is all about accumulating knowledge with consistent transparency -- all useful information is accessible to all players.
Being in a premade is an undeniable advantage, and it should really be visible to all players.