AJ
73 Bevaria
76 320i
77 320i
79 320is
80 320i
81 320is
82 320is
87 L6
99 323i

It's pretty easy to make a 320iS look-alike. The major differences are as follows:
-Recaro interior
-Limited-slip differential
-13in. basket-weave BBS-style wheels
-rear designation delete
-rear sway bar (16mm)
I'm not sure if front/rear spoilers were optional or not, and there are rumors that the suspension was sport-tuned, but I've never seen conclusive evidence of that. "s" models also came standard with many optional things on a regular 320i - sunroof, dual power mirrors, fog lights, and some other things. I'd look for most of those things, and if you have the majority of them, it'd be safe to say you have a real 'S.' I'm sure someone knows if there's a code diffrence in the vehicle number, but I thought I'd ramble on for awhile. Hope it helps!
-alan
83 320i
79 320i
74 02