That isn't a GPRS modem. It's a satellite transceiver.
Generally, if you are using a terminal emulator, it should send each character as serial ASCII as you type it.
You could do a loop back test. Connect RX to TX on the serial port and see if what you type in the terminal emulator comes up on screen. If yours screen has what you type anyhow you should get each character twice.
Examples:-
If you type "abc" with nothing connected, and see nothing on screen, when you connect RX to TX you should see "abc"
If you type "abc" with nothing connected, and see "abc" on screen, when you connect RX to TX you should see "aabbcc"
That indicates whether your serial emulator is working.
At a voltage level, you should see around 3 V on the TX line, which should dip slightly as you send characters. If you slow the baud rate down as far as it will go, your voltage dip becomes more obvious. If you have an oscilloscope, you should be able to see the individual bits of the ASCII character.
Does the transceiver have a command set like a modem has? Are there simple command that you can use to check communications?