The easiest way to tell that a tree is a Douglas-Fir is to look at its cones. If there aren't any on the tree, they're probably lying on the ground all around.

Underneath each of the scales of a Douglas-Fir cone is a funny-shaped lesser scale, known as a bract. Each bract has a long central stalk with a pointed wing on each side. Each bract strongly resembles a little mouse trying to escape into the cone. Pines, spruces, hemlocks and firs don't have these bracts; larches have wingless ones.

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If for some reason you can't find a cone to identify the tree, pull a needle off one of its branches. A Douglas-fir needle will have a small foot (called a petiole) at the base, and the branch will have a small raised area where the needle was. In contrast, if you pull a needle off a true fir, it will be the same thickness throughout and the branch will have a small depression.