The 48SX/S doesn't have a VERSION command, but one way to find the version is to press and hold ON, press and release D, and release ON to get to the interactive test screen, then press the backspace key, then press EVAL. This briefly shows you the version. EVAL can be pressed again to see the version again. When you're satisfied, invoke a warmstart by holding down ON, pressing and releasing C, and releasing ON.
Another method is to invoke the SysRPL command VERSTRING. This can be done by executing #30794h SYSEVAL. This will return "HPHP48-x", where "x" represents the version letter. The usual dire warning to check that the binary integer (including the trail h for hex base) are correct apply.
Yet another methocd is to transfer any object, in binary transfer mode, to your PC, and look at the resulting file with a text editor. The first eight characters of the file will be "HPHP48-x", where "x" represents the version letter.
Regards,
James