EPA has a radiation monitoring system, Radnet. Monitors are located as shown on their map:
EPA's Radnet Monitors (The Shade of Blue Indicates Whether the Monitor is On, Off, or Under Review)
Updates on Radnet results are summarized here by EPA; for example, yesterday they wrote
As of 1:00pm (EDT) EPA’s RadNet radiation air monitors across the U.S. show typical fluctuations in background radiation levels. The levels detected are far below levels of concern.