Are you looking for a remote solution or an in house test solution?
Perhaps a simple in house solution would be to remove two of the XBee-Pro units from your products and connect each of them to a XBIB-U board.
You could then use the range test feature in our XCTU program and perform a range test by powering them up and comparing the results to your known good antennas.
Obviously if the antennas are failing you would see very poor RSSI figures.
You can see an example of the range test feature in the video here http://youtu.be/EA-2Xa5OAY8?t=2m10s
You can find out more about the XCTU program here http://www.digieurope.de/products/wireless-wired-embedded-solutions/zigbee-rf-modules/xctu
Your account manager should be able to get you pricing on the XBIB-U boards.