Ummmm.... An Intercooler is a dead giveaway.
That is really the only surefire way. The brakes and callipers were bigger and fixed on the TT, but sometimes N/A's swap.
Stock rims were 17 on the TT vs. 16 on the NA, but again the aftermarket rims kill that clue.
And probably 75% of N/A's also had the wind installed as a dealer option so scratch that.
The turbo decal was also an option on most years, but I think standard on 97+
So - back to the original comment, and intercooler is the only way you can really know for sure.