I recently put up an upside-down suet feeder and have patiently been waiting for its first customer. I have a traditional type bird feeder nearby that has quite a traffic jam in the morning. Since I have low cost access to suet I thought that a suet feeder would be a nice addition to the bird feeding scene. Below is a picture of the only bird feeding on the suet. He is doing quite a job on the suet! I think it is a male Hairy Woodpecker rather than the Downy Woodpecker because of the beak and the red feathers on his head. Here is a description from Cornell’s All About Birds page on Hairy and Downy Woodpeckers, Visual comparison of Downy and Hairy Woodpeckers.
Overall size and bill size and shape are the most useful characters for distinguishing the two species in the field. There are subtle plumage features that can also be useful when a good close up view is possible.