A good way to tell if a player is good or bad is to take a look at their lolking stats.

If they have NOTHING BUT champs with average KDAS such as 7.1/6.9/5.5 or 9.2/8.5/6.7 (1:1 ratio), that's usually an indication that they have no idea how to play the game. Excluding support classes of course. It doesn't matter if they're Bronze 5 or Diamond 5.

Edit: Thanks for the downvotes. The only people downvoting me are the ones with piss-poor stats.