It's a dumb question. Once I knew this command, I used it two or three months ago but now it seems I can't recall it. I tried to look for it using Google but nothing happened :(
It's a dbus command which send a query to udev or something similar, basically I could do
mysterious_command | grep Storage
and I could see all usb storage drives.
(No, using mount it's not the same because it just gives me mounted partition and not if they are usb storage or whatever...or maybe it can and I didn't know about it).