Buy a cheap metal detector from a local retail store, such as Wal-Mart or Sears. It shouldn't cost you more than $30 or $40. The more expensive models have extra features that are only useful to treasure hunters.
Lay the old wood out on a flat surface, such as a work bench or the floor of your garage. Turn on your metal detector, and move it slowly over the old wood. Flip over the old wood, and move the metal detector slowly over the other side to find additional hidden nails. The metal detector will beep when old nails hidden in the wood are located.
Scan all of the wood before you begin to remove the hidden nails. Mark the location of each nail you find with chalk. Remember to wash off the chalk once the nails have been removed.