when the adapter is inserted so I guess support for it is there, so in theory it should work, but how do I actually test it?
I thought I'd dig out my old USR Courier HST modem to see if any lights on it could be flashed using minicom....but it has some strange power connector which I couldn't find. Maybe I'll try using my old LapLink cable if I can find it...
You can do a serial loopback test by connecting the RXD and TXD pins of the RS232 connector (IIRC pin 2 and 3, but please double-check beforehand).
If you then connect to the serial port (usually /dev/ttyU0) with e.g. cu(1) (or minicom, screen, ...) you should be able to see everything you type as it is sent and immediately received.