I highly, highly dislike overly artificial “firewalls” as +Topher Gerkey put it between in character and out of character knowledge. Yes, I expect that for the most part players will refrain from blatantly using their own knowledge to the detriment of everyone’s enjoyment of the game. But, conversely, I expect that they won’t stubbornly refuse to use it when it adds to the game.
As an example, since I’m gearing up for Tribe 8, is how I’ve seen portrayals of characters in post-apocalyptic games finding relatively common objects like flashlights. Usually it goes down as the GM describing the object in vague details, like “It’s a metal cylinder with a piece of glass set in the end.” and sitting back while Twenty Questions ensues and the GM tries to lead them astray from the real answer as much as possible. If asked to draw it or show a picture, the GM hems and haws and doesn’t want to because they know the players would immediately figure out it’s a flashlight.
Look, just say it’s a fucking flashlight. The players should be able to roleplay their characters figuring out how it works. The words, “What is this weird metal cylinder with glass set in the end?” should be coming from the players and not the GM trying to play some kind of lame game of Charades.
So, to that end here’s a quick flow chart of how I make these kinds of decisions: