gst-device-monitor-1.0 is a command line tool that can be used to test GStreamer's device monitoring functionality. By default it will just print a list of all devices found and exit, but you can also make it wait around for more devices by passing the --follow option. You can also pass it one or more filter arguments to filter the kind of devices you are interested in.
8 --help Print help synopsis and available command line options
8 --version Print version and exit
8 -f, --follow Don't exit after showing the initial device list, but wait for devices to be added or removed
8 -i, --include-hidden Include devices from hidden device providers.
8 gst-device-monitor-1.0 --follow
8 gst-device-monitor-1.0 Audio/Source
8 gst-device-monitor-1.0 Video/Source:video/x-raw
8 gst-device-monitor-1.0 -i Audio/Sink