If you want to narrow down the year of manufacture, you can date the components by using this Vintage Trek site. On the back of some of the components, such as the crank arms and brakes, will be a date code. This is not always that exact, because the components may have been replaced, but if not, you can figure that the bike was sold within a year or two of the component date.
http://www.vintage-trek.com/component_dates.htm