The M3 shares few major parts with the rest of the E30 range...
All the body panels other than the bonnet, front panel and sunroof are different (although the doors aren't too different)...
In particular, the front and rear arches are flared..totally different to every other E30.
Then the roof is reprofiled at the rear to change the angle of the rear screen.
Then the bootlid is different and the M3 one is, uniquely, in GRP...Spoiler attached is also unique, but varies depending on the M3 variant...
The engine and running gear are different, although the 320is shares the engine type (but not displacement)....
Lifting the bonnet should give the best idea
an M3 will look like
a 325, for example looks like
But you could also check the running gear...All M3s have 5 stud hubs...all other E30s (including the 320is) have 4 studs...
All M3s have a dog-leg 5 speed box
The anti roll bar mountings are different on the front and, unless it is a conversion special, it will be LHD...
There are plenty of other detail differences too...
Rear seat is unique to M3s, for example...
Front and rear screens are bonded (unique, too)
etc, etc...
