The barrel should have a length listed, it might say
3" Mod
or
2 3/4" Full
or even
3 1/2" Magnum IC
It will list the shell size, and the choke. Just about all modern barrels will handle a magnum load, but check the papers that came with it anyway. The big question is whether the receiver was a magnum model, since it's older. If you call Mossberg with the serial number, they should be able to tell you what it's rated for.
Truth be told, though, unless you're going turkey hunting (which most do not use an 18.5" barrel for), I'd be hard pressed to find a reason to use magnum shells. My 500 is Magnum rated, 3" chamber, but I've never had occasion to need one. 2 3/4" reduced-recoil 00 buckshot will kill an ox if it's inside my house.
Don't worry, even if the receiver's not magnum, you're not depriving yourself of anything.