How do I know it is a Swainson's Hawk?
FOSH Board member Chris Conard has prepared a two part briefing paper comparing Swainson's Hawks with other common raptors of the Sacramento region. You can download that Word document in two 4-5 mg files here. Raptor doc1 (covers Swainson's and other buteos). Raptordoc2 (forest hawks and No. Harrier).
Below are some highlights - photos of hawks sometimes confused with Swainson's Hawks.
Cooper's Hawk (l) and Red Shouldered Hawk (r) (photos by Ed Harper)
Red Tailed Hawk (l, photo Rebecca Houseman) and Northern Harrier Female (r, Photo Dan Tankersly)