Monitor 136.725. It's the Presidential Air Fleet Support frequency. You'll hear the lead aircraft generally giving flying conditions and weather to AF 1 along the flight route and also the conditions on approach at the landing airport. There is generally a lot of chat between the pilot of the SAM lead flight and the pilot of Air Force 1, especially as they get close to their destination. I don't think I've ever heard AF 1 actually identify himself on that freq - but it's him. The speaker aboard the SAM flight always refers to the other pilot as "Sir" which is an excellent tipoff. That freq is always the tip I look for when they're coming home to Andrews AFB.
One other note. At least at Andrews, the pilot aboard AF 1 doesn't call on a freq and wait for a response before giving his message. He just gets on the freq, says what he has to say and and the ground controller had "better" answer up immediately.
Alan