Lumber that is going to stay outside in the elements, exposed to moisture and insects, requires a preservative to keep it from prematurely decaying. Lumber manufacturers infuse preservatives into lumber in a process called pressure treatment. But not all lumber is pressure treated. You may need to tell the difference between pressure-treated wood and regular wood if you wish to dispose of the wood. Due to the preservatives, many municipalities will not accept pressure-treated wood for disposal. Additionally, you cannot safely burn pressure-treated wood because harmful chemicals in the preservatives become airborne as the wood burns.
Look at the cut edges of the wood to note the colour. If you notice a green tint to the wood, assume the wood is pressure treated. Be aware that as pressure-treated wood ages, however, this green tint fades. Green-coloured wood is a reliable indicator in new lumber, but aged lumber often becomes a weathered grey colour, with the green colour difficult to detect.
Examine the surface of the wood. If you find many small slits along the wood surface, conclude that the wood is pressure treated. Lumber manufacturers use the slits to help the preservatives absorb into the wood.
Consider the location of the wood and note the condition. If the wood has been outside in the elements for a number of years without significant decay, it is probably pressure treated.
Err on the side of caution when trying to identify pressure-treated wood, especially if you plan to burn it or dispose of it. Assume any outdoor wood is pressure treated to avoid mishandling pressure-treated wood.
Wood without pressure-treating preservatives will decay and decompose quickly in outdoor locations.