The Pencil Trick for Planing Boards
Before face-joining a board, mark it up with a pencil. However, don’t just draw a squiggle down the board, use a pencil to draw straight horizontal lines from edge to edge all the way down the board. By doing so, you will ensure that the edges of the board are square as well. Each time you plane the board in the joiner you will be able to see how flat it is based on how many pencil markings are left. Continuing face joining until all of the markings are gone to make flat boards.
To learn how to make more accurate measurements learn how to check if a square is square.