There is very little difference between the 2. Most notable will be the barrel and magazine tube. On the 590, there is a ring on the barrel that slides over the mag tube with a detent ball on the face towards the muzzle. The magazine cap comes off completely, and when screwed on, locks up against the detent ball. The 500 has not so much a ring as a retainer for the magazine cap/screw. Additionally, the mag tube of the 590 has a "cap" at the end of the tube which can be seen after the mag tube cap has been removed. The internal cap has a hole, and a bit of a slot. It too can be removed so the mag tube can be cleaned.
All that make sense?
More differences come in with the 590A1. Heavy wall barrel, aluminum alloy receiver, trigger gaurd, and safety. And the receiver is stamped "M590A1".
Well I'm thoroughly confused now! lol