Not sure of a difference on the VIN, but in the appearance department, look for:
SS badges (obviously)
17" 5 spoke rims
LT1 under the hood (available in the non SS car also)
body colored bumpers and trim.
check also for
rear spoiler, ss seats, all ss impala badging...on and on.
the impala was only offered in one color for interior-grey. and the headrests are badged SS.
1994 impala ss was only offered in black. 1995 and 1996 were offered in three- dcm(dark cherry metallic) bbb(black somethin somethin) and DGGM(dark green grey metallic) all with the 17's with the 5 spoke.
there are several impala specific ways to tell. 4 wheel disk is offered on impalas and not on civi caprices and 9c1s.
on vehicle SPID in trunk, there will be a WX3 label in all those number codes. check the vin on the SPID and on the dash and make sure they are the same.
those are the most prominent ways. thats all i can think of now, hope i helped you out.