Is there a way to watch service calls?

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One of the features I love about ROS's publisher-subscriber design is that I can passively "tune in" to the streams of communication between nodes to better understand their interaction. I have yet to find a convenient way to do the same for services.

I'm working on writing a man-in-the-middle node that could allow me to do this, but this seems like something ROS might provide already. Another option that I have yet to explore is using a debugger, but this could become tedious for a large software pipeline. Does anyone know of something better?

If it informs your answer, I'd like to use parts of the arm_navigation pipeline with existing motor controllers that do not conform to ROS's PR2 controller interface. Thanks