It's written on the long APT/WEFAX-II Adapter, but you would have to take your CPU apart to see that. The ANALYSIS software can be forced to read the serial number and display it at the top of the opening RESTRICTED RIGHTS legend. The steps are as follows.
Select FAST!:CAPTURE, then Unload Capture TSR.
When WTMAIN loads without the WT.COM capture TSR being present, it reads the APT/WEFAX-II Adapter Serial Number and displays it at the top of the Restricted Rights page. This will be a 4 digit number, like 1215.
You can then reload the capture TSR using Fast!:Capture, via Schedule:Go, or with the standard AFMIT configuration, by either rebooting the system or rerunning AUTOEXEC.BAT.