Steering wheel of an 87 would be textured plastic vs leather wrapped in a 91.
91 would have the Becker Gran Prix 1432 with separate amps and prewiring for or equipped with a Silverstone CD changer.
I think the 87 had 7" ET 25 wheels and the 91 had 6.5" ET 21.5 wheels. Same 205/65-15 tires. Really hard to tell the difference without pulling the wheels.
Passenger side airbag might have been standard equipment on a 91 560SEL.
I know I'll regret asking but why do you want to do this?