This is a neat tech that I didn't know existed until last week: http://ufis.ca/products-services/picus-sonic-tomograph/

It measures the time sound takes to travel between tissue types, with the density of wood reflecting its health and the speed of the sound. If you suspect the inside is decaying, you ring the weakest point of the trunk with nails and then measure those with a set of digital calipers. This gives the main unit a sense of the geometry of the trunk. You then use a hammer to strike each nail a few times while it detects the sound on the opposite nails with a set of probes. After circling the tree, it builds a precise estimate of the severity and geometry of the dead wood. Do it in several spots and you've made a CT scan for a tree.

On the first image you can see the that the trunk is deformed with a cavity on the lower left. This is a lateral view of the top of that cavity where the trunk faces the greatest wind stress. The second image shows decay in increasing severity with purple, the margins of that decay with green, and healthy wood with brown. I think blue is heartwood.