I'm a retired Wildlife Biologist with degrees in Forestry and Wildlife Science from Purdue University. Like another wrote, getting a book written about the trees in your area is your best bet. The book should cover leaves and very importantly, branch ends (twigs) also. Twig buds are one of the very good ways to identify a lot of different trees expecially when the leaves are off. Once you develop an eye for it the shape of the tree and the bark will be all you'll need for a lot of them.
We had a great Dendro Text and Key when I was at Purdue. By the way, those books would be solid for IL also if you can't find anything your University uses.