how to know sms recived in gsm sim900a

my project based on security system ;
i programmed to recive message when motor start and got output for that now how to switch off the motor when replayed to that message;
i required at-command for that and need program in c

What have you actually got working ?
Have a look in our GSM forums for similar questions and answers.

if you know how to turn on the motor then you should know how to turn off the motor. and i did not understand this statement:

"when replayed to that message;"

did you mean replied?

for 8051 i need program to switch off motor

ajay bhargav : when i press switch motor will run and gsm modem will send message (motor turn on)to number which is stored on gsm ,then when i replay to that message motor will stop:
i got output (for when i press switch motor run and gsm send message to me)
now i need to know how to program to switch off motor when i replay to that message
and i know at+command for message indication (AT+CNMI) but i did not know how to program

help me to complete this project

You need to send read command "AT+CMGR" with location where SMS is stored. Now there are two things you can do.
1. always poll first location for new SMS by sending "AT+CMGR=1" if sms is there you will get a reply with +CMGR else you will just get OK from modem. one SMS is read and processed you can delete it.

2. read the new message indication.. "+CMTI: "SM", 1" this should be done using UART interrupt as its an unsolicited response from Modem.

We have a "GSM notice board project" which shows how to capture +CMTI response, and search in GSM forum you will find the other ways too.

If you are new to all this.. then I would recommend using first method.