The difference is that Full pressure turbos have higher boost, so higher
horsepower and torque.  Low pressure turbos do have less boost, but tend
to develop it at lower rpm, so the boost has less "lag".  Many people
prefer the LPT to the FPT, though I have 2 FPTs and am a torque junkie.
 I love to feel the rush of the boost coming on!  ;-)

As to how you can tell, if there is a boost gauge in the dash it's a FPT.

Here is a place you can get some past model year info on SAABs.
http://www.saabnet.com/tsn/models/

and here is the link to the 97 model year cars:
http://www.saabnet.com/tsn/models/1997/

It appears that in 1997, the 900s was available only with the normally
aspirated 155hp 2.3L 4 cyl.  The SE was available with either the (much
maligned) 170hp 2.5L GM V6 or the (much preferred) 2.0L (FPT) turbo
making 185 hp.

So contrary to my prior answer, in 1997 there was a difference in
engines.  In 1998 both the S and SE were available with the FPT 2.0L 4
cylinder.  I know I have one of each!  ;-)  A 3-door S and a 5-door SE.