Trying to find out if a deck has been treated in the past may become tricky. Unless the wood has an obvious finish on it only a professional can determine if the wood has been sealed. Things are not always what they seem. That’s why it is important to have a professional evaluation. You also have to determine how old the stain/sealer is if applicable. One way to find out if a deck has been sealed or stained is to pour a little bit of water on an area of the deck and see if the water beads up. If it does, then it probably has a sealer on it and must be stripped completely.