lsusb
This will show you all USB devices connected. Also, if you need to check if an USB device was plugged or not (if the ESXi can detect it) you can do:
tail -f /var/log/vmkernel.log | grep -i USB
And monitor it for the output.
Also, have in mind ESXi 5.0 does not support USB 3.0 devices/ports. Check http://kb.vmware.com/kb/1022290
Hope this helps 