Hold your material on edge and use a pencil to mark what you think is center.
Note: If your stock is too wide for easy guessing, take a small piece of wood and flush it to one side of the stock edge. Put a pencil mark on the stock toward the center using the wood piece as a rule. Flush the same small piece of wood on the other side of the stock edge and mark on the stock toward the center using the small piece of wood as a rule. Because these marks are close together, it is easier to guess at the center between the two marks. Mark a center spot on your stock between the two marks.