Ideally, you would use a spectrum analyzer to measure the power transmitted from one antenna. Then using the power you can figure out the range. If you don't have access to a spectrum analyzer. You'll have to use the age old method of trial and error. Moving the devices further and further until they drop the signal. Unfortunately, I can't tell you how far this is. The theoretical outdoor line of sight distance for Zigbee PRO is 100m however its closer to 10-25m depending on your setup.
XCTU will only be able to tell you information about the network. At best you could get LQI data outof the packets but even that wouldnt help you.