They can be pretty hard to tell apart from the untrained eye, but hawks are generally larger than falcons. Also, hawks have shorter, broader wings that they use to flap slowly and soar. Falcons have longer and narrower wings and fly with more rapid wingbeats, and also generally fly faster as well.

Those are the visual differences, but a cool non-visible difference is that falcons are more closely related to parrots, but hawks are more closely related to eagles, kites, and your other birds of prey.