On 19 Oct 2005 19:22:05 -0700, "nireedmot" wrote:

How can you tell exactly where along the 17" of joint
the problem is when nothing's visible?

I have an engineer's (metal machinist's) depth gauge. It's a chunk of
thick steel with a thin wire that slides vertically through it, with a
small clamp. Excellent gadget for probing how deep mortices are clean
to.

Another useful gadget is an internal caliper with a screw thread
adjuster. Hard to use as you need to hold it square, but it will often
pick up a narrow spot that needs paring.