The USB Armory comes with a nice little LED on board. In the default Debian image, it is set up to blink based on CPU usage. To control it manually, take the following steps:

The default image for the USB Armory defaults to using the g_ether driver to provide an ethernet device to the host. However, that does not give you a USB mass storage device. The alternative is to modrpobe -r g_ether and modprobe g_mass_storage file=/path/to/block/device, but that leaves you without ethernet, and thus without a method of communicating with the armory.