Find associated USB port with either lsusb or dmesg. Example of lsusb output:
$ lsusb
...
Bus 001 Device 011: ID 047d:2048 Kensington Orbit Trackball with Scroll Ring
Set a filter with corresponding bus and device ID, i.e. usb.src == "1.11.1". When you move the trackball you should now see a lot of USB communication.
