Identification of raptors in flight, especially at a long distance, is quite a challenge, and it takes a few years to become proficient in identifying migrating raptors. Below are variety of tools available to help both novice and experienced hawk watchers with the identification of migrating raptors. Click on images for links.
Our recommended raptor identification charts:
A Guide to Hawks Seen in North America – This handy, 2-page pocket guide from HMANA outlines key fieldmarks and shows nice side-by-side comparative drawings of raptors in each family.
Hawk Identification Chart – This 2-page table provides succinct descriptions of raptor fieldmarks along with flight and behavior.
Raptor Silhouettes – This chart (courtesy of Acadia National Park HawkWatch) compares representative silhouettes of the major raptor families seen in eastern North America. This is a terrific and simple identification aid for raptors in flight.
Some highly recommended raptor field guides:
Hawks in Flight: The Flight Identification of North American Migrant Raptors 
Hawks from Every Angle: How to Identify Raptors In Flight
Hawk Watch-A Video Guide to Eastern Raptors (DVD) by Richard K. Walton & Greg Dodge. Superb videography and instructional narrative are combined to provide an effective method for learning hawk identification in the field. 2010.
Raptors of Eastern North America by Brian K. Wheeler. Abundantly illustrated with hundreds of full-color high-quality photographs. 456 pages, 2007.