The test circuit shown on p6 of http://www.radiometrix.com/pdf/tx2rx2.pdf shows how to use the CD pin. You'll need to modify your receiver schematic to add the transistor, resistor and cap. Then, just monitor the /CD signal to determine when a signal is being received. Depending on the response time, you may need to prepend a non-data byte to all messages. FSK signalling is much more reliable than ASK. If you add manchester encoding to FSK, you get a really robust system.