A recent banding program at Springbrook yielded the opportunity to have a Hairy and Downy woodpecker in the hand at the same time for some nice comparisons. The size is of course the first detail - Hairy woodpeckers are much larger. Their bills are also much longer in proportion. And the red feathers (both males) do not make a complete span around the back of the head like a Downy. Downy's also have bars on the outside retrices (tail feathers) and Hairy woodpeckers do not.
Never pass up a chance to post pics of a Fox Sparrow.