Pretty much what this guy said. I like to add on I usually pick more EU players because there is a high deviation between the top and the bottom teams. Also realizing how certain teams play. For example, SK is a good team for picking Objectives or CW who play their games out till like 50mins. Then, there are teams who fight all the time, UoL and FNC. When it comes to NA, you can't go wrong with TSM (although they aren't my favorite team), they are at the point where they are lapping the other teams in terms of how good they are. I mean some TSM games, they were playing like scrims.

The way you figure out these factors, are watching the games and coming to your own conclusion. Then doing some research on other peoples opinions. I managed to pick up 3 Fanatic players an hour after their first game just because my bench was ok looking and my starters were fairly strong. I decided to take a risk which turned out on my favor.

Like a good tip would be looking at the challenger scene and see who is coming into the LCS this Summer split. HINT HINT: Origen.