What do you mean by radio sensing a channel? I am slowly getting into the field of wireless communications, in particular with WiFi based communications. 

802.11 MAC mentions that 


  A station wishing to transmit first listens to the medium for a predetermined period and If it senses to be IDLE , then it transmits . If the medium is BUSY , it waits for some random time  and then again senses the channel


I have two questions here:


What exactly do you mean by SENSING here ?
I have a radio A which wants to transmit on 2.45 GHZ. Do I transmit anything dummy data to know if its is buysy or idle ? I am thinking of programming wise
how does radio A knows that channel is busy or idle ? Assuming there is no ACK concept here ?
The station here is continuously monitoring the energy in the medium. Let us say you want to transmit in channel 1 of 2.4GHz band. Your RF hardware is always in receive mode if you don't want to transmit anything. You are continuously sampling the incoming signal in this particular channel. You use ADC to sample and compute the energy $E = \sum |x[n]|^2 $ of these samples corresponding to window of say, 100us. This is a simplified concept of sensing. You don't need to transmit anything to sense the channel. The value $E$ is computed in terms of $dBm$ unit.
After the above step, the $dBm$ value is compared against a threshold specified in standard. For example, let us say the minimum sensitivity needed for receiving QPSK modeulated data is -60dBm. You need to make sure that your transmission is above this. So you sense the channel. If the RSSI (that is $E$ computed above) is more than -60dBm, then you cannot transmit as the channel is BUSY. If it is less than -60dBm, you can transmit. But to be on safer side you can give a margin of say 5dBm. For higher modulation order (like 256QAM) the threshold will be lesser as you need to have very less ambient noise while transmitting 256QAM signals.