You can't really tell just by looking at the antennas, unless you are familiar with who uses what kind of antennas exactly, and even that is difficult. Where I live there are 3 operators, and one of them owns the fixed-lines network, so I can usually tell which antennas belong to them because the other 2 operators usually have microwave dishes on them for backhaul.
RobSaah, As already mentioned, it's difficult to tell the players by their equipment alone. Now at AT&T is using Air 21 antennas, you can't assume that it's a T-Mobile site. What I do is to look for the door/fence sign that give the NOC phone number. You can tell a lot about ownership from the signs. Good luck! Jonathan