How to Poll USB mouse through terminal in El Capitan
For any regular USB device, use:

diskutil list


This brings up the disk utility command line interface's "list" tool, which provides you with information on internal and connected disks, be it through USB, SATA or PCIe. It's much simpler than looking through /dev (which includes ttys and a lot of other stuff), and gives you much more information like capacity, partition tabling and the S.M.A.R.T. status (if supported) of your drive.

If you'd like to look through /dev though, try

ls /dev | grep disk


This will show only the entries with "disk" in them (disks and connected devices in OS X are mounted as /dev/disk[number]).