Peregrine falcons have been clocked in screaming dives at well over 100 miles an hour. If you see a hawk moving in a straight line very rapidly, with strong, quick wingbeats, you are likely watching a falcon. To be sure, try and glimpse the wings. If they are long, angular and pointed on the ends, that is a tell-tale sign. Most of the other hawk families have wings that are wide and round on the ends, with few exceptions.
Long tails are another good field mark.
A. Falcons we have in our area during migration are the American kestrel, the merlin, and the peregrine falcon. Although your most probable view of a peregrine or merlin will be while the bird is in the air migrating, kestrels can be observed year-round. Like all falcons, it prefers wide, open spaces with good perches from which it can observe any movement. For more detailed information, consult either the Peterson's Guide to Eastern Birds, or, more specifically, the Peterson's Guide to Hawks.