Ive been shopping for boxsters around and I found a 1998 boxster at a used car dealer near me claimed to be an S. The back just has the S letter offset to the right like it would be on an S car. It has a redish interior and some different looking multi-spoke wheels. Here is a picture.

I dont know boxsters all the well but I dont recall a 98 S. Something up here?

 

The first-generation Boxster (the 986) was introduced in late 1996 as a 1997 model; it was powered by a 2.5 litre flat six-cylinder engine. In 2000, the new Boxster S variant was introduced with a larger 3.2 litre motor, and the base model received a more powerful 2.7 litre engine.





Porsche 986


Porsche

Production 1996–2004 Engine(s)
2.5 L flat-6 (1997-1999)
2.7 L flat-6 (2000-2004)
3.2 L flat-6 (2000-2004


Does the car have a bigger engine in it? I would ask that. Maybe that's why the "S" is on the car. I would want to see the car records and check out the VIN number to verify when the car was produced.
http://www.986faq.com/1-0/default.asp#012
Model Year V=1997, W=1998, X=1999, Y=2000

Your comment on the wheels is not the determining factor if the car is a "S". It is not uncommon for Porsche owners to change out their wheels.
Good Luck

 

Not sure about the early year model Boxster, but I know 2003, 04, 05, 06, 07, the tailpipe on the regular Boxster has one hole, the S have two holes. May be you can check it out.

 

Another 'S' factor for Porsche are the red brake calipers but like stated earlier, wheels and brakes can be swapped-out pretty easily. Wikepedia is another good resource http://en.wikipedia.org/wiki/Porsche_BoxsterGood luck.