I picked up a Mossberg 500A (7 plust 1) a few years ago, and so far all I have shot out of it is birdshot.  I have no interchangable chokes, and I cannot see any markings stating what cylinder I have.  


Can I shoot slugs through it?

Check along the left side of the barrel just ahead of the receiver.  There should be some marking as to the choke.  Probably "Cyl" or "Imp Cyl".

If it is a Home Defense style gun, it's surely one of the two, and therefore perfectly safe to shoot slugs out of without fear of damaging it.

If you have serious doubt, get a patterning target and pattern it

Also, you can take it to a qualified gunsmith, and he can check the choke with a guage that measures the actual constriction of the bore.  

most likely the answer is yes.

When you pull the barrel. Is there anything marked on the chamber hood? If its a stock barrel I would say ya it should be good for slugs. But check the chamber hood for markings first.