Using a home made air knife with a trailer mounted compressor we did the root crown inspection.
Almost straight away we found older fruiting bodies that had become buried under grass clippings and slight soil grade changes over a number of years, this fungal decay has been going on for quite some time in this tree.
As the excavation continued we became more and more aware of the extent of the decay and the ease at which the hand saw goes into the stem 300mm is certainly a worry!
Now you could argue why not do the saw into the tree test before all the expense of the air excavation, and I'd have to admit I should have! I just didn't think the decay as quite that bad until the soil started to be removed, lesson....always have a probe of some sort to carry out this kind of simple test, the same probe for finding the root crown in fact.
If you had the probe clearly striped black and white in 10mm intervals it could also be used as a scale indicator in pics (three tools in one!)