For details about AT commands refer this link
If you are using windows you can use hyperterminal to communicate with any modem. In this case the GSM modem. Find a method to connect your GSM modem to the PC (Refer this Link
). You can use a USB2TTL converter to directly interface your GSM modem to PC. After interfacing the modem go to device manager and find the com port no: assigned to your GSM modem.
After identifying the port number open hyperterminal. You need to download Hyperterminal or copy the files from windows XP. You can also use putty for this purpose.
Type a name and click OK. Then select the com port you are using and press ok.
Change the boadrate to 9600(you can choose other also. Higher boadrates may not work with some GSM modem) and let other options remain unchanged.
After that type AT and press Enter. If you get a response showing OK. Then everything is fine. As a default typed character are not shown in the terminal. You can enable this by going to File=> Properties=> Settings=>ASCII Setup and check Echo typed characters locally.
You can follow the same procedures in putty also. This is a screenshot of putty.
You can do the same in Linux also. Use minicom or any other terminal softwares for communicating with you GSM modem in Linux. This link describes how to use minicom.
And this link describes how to identify the name of your modem.