(Almost) surefire way to know if someone is going to carry or not

Yeah, it's not completely fail-proof, but it gives you a very good idea.

LOLnexus.com or any other related side. Right as the game is about to begin.

There's a Platinum IV Khazix jungler with a 7.1/7.8/x kda and 19 games played? He has 700 games and he's JUST Plat IV? You can bet your bottom dollar he is absolute trash, will go to mid lane after enemy LeBlanc got a kill and will take the CS etc and die in the process. You can also bet he is going to try to 1v1 the Lee Sin who has 2 levels above him.

What I do when I jungle is check who from my team has the most potential to carry (KDA wise) and put my undivided attention there. Camp for them, get 'em fed, tilt the enemy laner, proceed to win the 2v8 game. I'm lucky if I ever get 2v8s. It's mostly 1v9s, I know it's a bit much to ask.