The keys to a successful Hawk Watch are easy. Sorta. First you study forms, you read books like Crossley's Raptor Guide (C) and Hawks in Flight (C). Then I think the best thing to do is to bird and watch in the presence of people who know what they are doing. The Connecticut Ornithological Association (COA) ran a great workshop and I am looking forward to more time doing a hawk watch. Once you are out there, you hit the real challenges. Raptors like to soar and glide around. The keys to identifying these raptors become Behavior, Color, Tail, and Technique. How can you tell the difference between a Red-Tailed and a Red-Shouldered Hawk when they are a mile high? Well, Wings, behavior and timing become the keys. Experience can teach you a lot about this. Although,we will all still be tripped up by the Male Cooper's Hawk and the Female Sharp-Shinned Hawks.
In the mean time, I suggest getting out and finding a Hawk Watch
A Raptor Far Off
Getting a Great Lesson from the Connecticut Ornithological Association
A Bald Eagle looking like a Chocolate Sprinkle
An Osprey and a Bald Eagle fighting it out
Hawk Migration Association
Hawk Watch International