Using signal strength feature of nRF24L01 module
To get the signal strength just send 100 packets with no retries and see how many times the packet arrives.
radio.begin();             
radio.setRetries(0,0); // by default nrf tries 15 times. Change to no retries to measure strength

char buffer[32];
int counter = 0;

for(int i=0; i&lt;100; i++)
{
   int status = radio.send(buffer,32); // send 32 bytes of data. It does not matter what it is
   if(status)
       counter++;

   delay(1); // try again in 1 millisecond
}

// if counter &gt; 80 signal strength is great
{

}

// else if counter &gt; 60 signal strength is good
{

}

// etc

radio.setRetries(5,15); change back to its default when done.

For this code to work the receiver must be listening for data. Just listen for data on the receiver there is no need to validate the data.