Tont already answered your question...
If it had a Injen Dress up kit then it should be a GTS because Injen does not make a kit for the GT
Another few ways to tell...
is your exhaust tip curved/oval or flat/round? (curved is GTS)
tach on a GTS is 9k RPM... GT is only 8k RPM
speedo on a GTS is 160mph... gt is only 140mph
rear brakes on GTS ar disk... gt has drum
engine on gts displays metal vaulves... gt displays plastic covers
gts comes with leather seating... gt is cloth seating
gts has side air bags on the seats... gt does not
gts comes standard with sunroof and power windows
there are several other ways to tell the difference, but after just looking at the above listing of obvious visual features, you will know exactly what you got.