You can't - and that's the short answer.
Unlike many of the other outboard makers, Merc does not "encode" that information into the serial.
With the serial many of us can tell you what year it was "produced", not the model year it was sold as. Merc (I personally believe) did that on purpose so that a dealer would never have "last years model" (identified by the serial) sitting on the floor. If it was built in 1986 but sold in 1988, it would have been sold as a brand new model.
There are a couple of dealers on this board that CAN run your serial and give you the model/horsepower etc but you will have to post up what you have...
PS - if you have one of the Yamaha built mariners, then that can be broken all out to give you the information you are after - again, post your serial and somebody can help you out..