How do you know which pawn has been promoted?

Whenever a pawn reaches the 8th rank (the end of the board opposite that on which you started) you can promote that pawn. This means that you trade out that pawn for any piece you want. Note that it doesn't have to be a piece you've already lost during the game...you could have two queens out at the same time if you want. While in most cases it is best to "queen" your pawn, it is sometimes beneficial to "underpromote", or promote your pawn to a piece weaker than a queen. This is often done to a knight, since it's the one piece who can move in ways the queen can't. It also is sometimes worth promoting to a rook, in cases where promoting to a queen would put the opposing king in stalemate.