You could try reading the RSSI register to see how strong the signal currently is, or check (PKTSTATUS & (1<<3)) like we do in radio_mac.c to see if the radio is in the middle of receiving a packet. I’m not sure how well either of those strategies will work. The basic problem is that it takes some time for the Wixel’s radio to change from RX to TX, and during that time its possible that another Wixel will start transmitting.
While I enjoyed programming the Wixels, I have since moved on to the Xbee Series 1 modules. You might want to look into these. Similar price, much better range and built in network that supports any node talking to any other node.
As far as the Wixel however, they don’t have a good range but are some tough little widgets. I’ve implemented a master/slave set of apps for them, if you would like some more example code to peruse there is a free download here: