Alright, so for starters this is my first time posting, and I love the game. That being said however my friends and I are having a little disagreement. One of us says that once a development is played face down you cannot look at it to determine what it is. I didn't find anything in the rules about it so I see no reason why you wouldn't be able to. Could someone help out with the clarification here?

A player may
look at his developments (but not at his opponent’s) at
any time.

Page 15 of the rulesbook.