Listening in on ANT+ device communication?
Hardware USB analyzers tend to be fairly expensive but for a device already working a software analyzer should suffice. The following would be some good terms to search for:


USB protocol analyzer
USB bus analyzer
USB bus sniffer


There is a question at StackOverflow with some answers that offer suggestions on some packages that are available if you're running Windows:

What (software) usb sniffers are available for windows?