sudo lsusb
Edit:
According to a blog post from Microsoft this feature is now fixed from build 16176 and forward.https://blogs.msdn.microsoft.com/wsl/2017/04/18/file-system-improvements-to-the-windows-subsystem-for-linux/
There is no USB support at the moment. Only fixed disks will be mounted (automatically) in /mnt.
Also other storage (other than auto-mounted fixed disks) are not supported, This includes CD-ROM drives, loopback devices and network shares.
Here is a link to the developer feedback page: Unable to access USB devices from bash