All swallows have longer tails, usually with a notch or fork, and broader wings. Black Swift and White-throated Swift have forked tails and different patterning below. Vaux's Swift is extremely similar but is somewhat smaller, with shorter wings, a paler rump and throat, and a different call.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.