As I wrote in another thread, the Mossberg 590A1s seem to have the six characters "M590A1" engraved on the left side of the receiver (opposite of the ejection port), in rather large characters. I am not sure about the 590s, though, but this should be a good indicator that it is a 590A1 (heavy barrel and steel trigger guard, and steel safety).
[Edited by lunde on 01-29-2001 at 11:45 PM]