You can try the following:

LOCAL oComm, i, cOldErrorHandler
DECLARE aComPorts[16]
aComPorts = .T.
cOldErrorHandler = ON('ERROR')
FOR i = 1 TO 16
  Wait WINDOW "checking Port "+ALLTRIM(STR(i)) NOWAIT
  oComm = CREATEOBJ('MsCOMMLib.Mscomm')
  oComm.CommPort = i
  ON ERROR aComPorts[i] = .F.
  oComm.PortOpen = .T.
  ? oComm.CommPort
  ?? oComm.PortOpen
  oComm.PortOpen = .F.
  oComm = NULL
  ON ERROR &cOldErrorHandler
ENDFOR
?
?
?
WAIT WINDOW "Finished!" NOWAIT

>How can I find out what comm ports are on a PC? I have a PC that doesn't have comm port 2 defined even though there is one on the PC. Any ideas?
>TIA!