If you’re lucky enough to live in an area where you have both downy and hairy woodpeckers, you may be hiking in the woods and see one of them. They look identical except for the fact that one is small and one is large. But how can you quickly tell who is who if they are not next to each other? If you do not have any type of scale for comparison?
Check its bill.
If you pretend to take the bill off and place it on its head, does it only cover a little portion of its head as in the downy woodpecker? Or does the bill almost cover the whole entire head as in the hairy woodpecker?
The downy woodpecker has a very small bill and small body. The hairy woodpecker has a long bill and long body. So if you see one in the wild, quickly look at its bill and compare it to its head size. There is your secret answer.
Happy hiking and painting!
Let's Paint Nature! 
It’s like choosing what restaurant to eat at: do you want to see a big bill or a small bill? : )
HA HA HA…. a small bill! A really small bill.