Do you happen to know the model of the headset? Many older Treks used Cane Creek headsets (possibly S2 - S3). Does it have a cartridge bearing or loose bearings? There should be some sort of model number embossed somewhere.

You'll probably find that if you install a headset spacer or bearing race upside down you'll notice some larger than normal gaps between the fork crown and the head tube. Also, it won't feel as smooth as it should. It's usually not that difficult to figure out. I mean, it can only install one of two ways.