Examine the color of the wood to narrow your choices. Light wood is usually ash, oak, birch or maple. Dark wood is often mesquite or walnut.
Touch the wood to determine how hard or soft it is. If you can scratch or dent the wood with your fingernail, it is a softwood. Softwood is generally cheaper than hardwood and is easy to nail so it is often found on decks. Pine, spruce and cedar are types of softwoods. Oak and beech are types of hardwood.
Study the grain of the wood. If the grain is tight and hard to see, it is a closed-grain wood. If the grain is large and easily visible, it is a open-grain wood. Softwood has a closed grain but hardwood can have open or closed grain. Oak and ash are open-grained wood. Maple, alder and cherry have closed-grain wood.
Compare the information you have compiled about the characteristics of the wood to an online database or book to determine specifically the type of wood used in the construction of your deck.