This report presents how Pandora, our flexible monitoring platform, can be used to monitor a system of cooperating proxy caches. It circumvents many of the drawbacks of currently used techniques: Pandora operates on-line, it does not depend on specific cache software, and it can be adapted to any system configuration. We also present two applications that exercise the tool. The first one extracts a Web access trace in presence of cooperating proxy caches purified from the biases they introduce. The second one is an evaluation tool that measures the instantaneous quality of service provided by a system of caches according to customizable compromises. Finally we show effective utilization of these applications, in a real environment for the pure trace extraction and in an artificial experiment for the second.