Important to note that, unlike with Goodyear welted shoes, the welt and upper are stitched directly to the insole. The holdfast is part of the insole, and is not a strip of canvas that is glued to the insole.
it's hard to draw a complete diagram, but carving the holdfast into the insole looks something like this:
Before the outsole is sewn on, but after the upper and welt have been stitched to the holdfast, it looks like this:
The welt running around the outside is stitched through the upper directly to that ridge, the holdfast of the insole.
Of course, once the outsole is sewn on, it's almost impossible to tell if the shoe is Goodyear or Handwelted, since there is no stitch that travels all the way through the insole in either construction.
However, handwelting the shoe - when you force an awl through the holdfast to make the holes - creates little dimples on the top surface of the insole, like this:
This is the big clue that tells you that the shoe is handwelted, not goodyear welted.
However, some makers will have a full sock liner, so you can't directly see the insole. In that case the only way of knowing for sure is to take off the sole and look.
I picked this all up from browsing MC. There's a lot of info there if you can wade through the trash.