You cannot prove full functionality of a modem without using a telephone connection (in other words, try it. If it works, it works, if not, it doesn't work). You can exercise the RS-232 serial port incrementally. Place a loopback jumper between pins 2 & 3 of the serial port D-sub connector. This will link the Tx signal back into the Rx pin. With any terminal emulator, press keys and observe that the keystrokes are read back to the terminal. If you connect a proper cable (normally a 'straight' cable) between the serial port and modem, you should be able to verify that the modem, serial port and cable are functional by observing that modem commands can be received, echoed, and interpreted by the modem. Again, this is done by typing modem commands in a terminal emulator, and observing the modem response to the commands.