Question
can an arborist x-ray a tree to see if it is hollow?

Answer
Yes and no. They usually use a machine that records the resistance of the wood and from that reading can determine if the tree is hollow or has decay present. Another way is to drill into the tree trunk with an increment borer and pull a core out and determine decay. here is some information on the Resistograph web site.  http://www.vermeeraustralia.com.au/brochures/resistograph.htm

There is also a unit called a tree radar that will also measure the decay. here is a web site on that.
http://www.treeradar.com/