Is there a reliable, non-destructive way to tell if a tree trunk is hollow? I have an oak that I’m considering cutting down because it appears to be unhealthy and possibly hollow. I had another one about 40ft away taken down that had a hollow trunk, so this gives me reason to believe the other tree is the same.

Bill,

A sounding hammer/axe or increment borer are two non-destructive methods used to determine "soundness". Both of which, I doubt, you have in your possession.

However, you can use the non-cutting portion of an axe to "sound" the tree. Firmly, but not destructively, tap the outside of the tree to determine whether or not you hear a hollow-like sound. This simple method is employed by most foresters and timber buyers when looking at standing timber for sale.

If you are worried about the structural integrity of the tree in question, then I suggest contacting a certified arborist.