Looking at the tracks will give you a pretty good idea. If the tracks are very shiny on the top, are welded (i.e., the individual sections of track are welded into on long continuous piece), and the ballast (gravel) is neat and weed free, they probably see a lot of use. If they are jointed rails (individual sections of track bolted together with plates on the sides), uneven, and the ballast has weeds, probably not.
Some tracks are seasonal. We have a number of tracks around here that only get used during harvests.
There are quite a few tracks that don't get used any more, but the ones that are used are used very heavily. Rail traffic has dropped off with the economy, but is trending back up. I've seen construction of new lines occurring here in some busy areas.