How to use SCPI on HP33120A via RS232?
The easiest way to debug SCPI over RS-232 is to use a terminal program with your existing serial interface. Just type the command and see what comes back. The terminal allows you to fairly easily fiddlle with baud rate, handshaking, echo and termination characters without relying on some unknown serial communication library in between (what you type is what you send, what is received is what is shown).

*IDN? doesn't need any special trigger or prompt for response. If the command is supported (according to the manual, it is), if the termination character is correct (which it appears to be, since your previous Python work had some success) and if the handshaking is correct, the instrument should respond immediately with the identification string.

Once you figure out exactly what the instrument expects and returns, you can then craft your Python script with ease.