All of Sprint's sites are labeled. You just have to know where to look and what to look for. I believe most of Cingular's and Nextel's sites are also labeled. Verizon and AT&T sites are usually not labeled and are more difficult to ID.
Don't really want to give away my secrets. Just kidding. Sprint has a certain type of label (radio frequency warning) that's on the base of the antenna itself. It will be blue and white in color and has a certain 888 Sprint phone number listed on it. This can postively ID a Sprint site. In situations where you can't see the base of the antenna then I just look at the site ID plaque posted on the enclosure. Each carrier has a different numbering method for their sites. In addition most of Sprint's sites actually have a decal somewhere on the equipment boxes that reads something like this: Sprint PCS network Operations. Hey 714tmobile why not stop by the new Cingular/Sprint shared site in Anaheim Hills at the intersection of Meats & Nohl Ranch Rd. You will see that both Cingular and Sprint appear on the telephone boxes at the bottom of the slope. The enclosures and panels are higher up on the slope but are easy to get to using the wooden steps. The brown panels are Cingular and the green ones are Sprint. You can also see the blue & white radio frequency label with the 888 number on the door of the Sprint enclosure