Quote - ..... what I was really asking was if there was a way of actually knowing or determining "which" polys should be grouped to "rThigh" and "which" polys should be grouped to "rShin"

Sometimes, it is obvious.  Sometimes, it is not.  As PhilC said, it may not need groups assigned to the shin even if it overlaps them a little bit.  IMO... it "Depends."

The only way to know for sure is to experiment.  If you group it and find that you just can't get good control over pesky problems no matter how much you spin joint parameter dials, you might have to regroup it.  For your kneepad, it probably isn't a big deal.  But, for something more complex, it could take a lot of tweaking.

It helps when modeling to keep in the back of your head the figure you're modeling for and how its mesh is grouped.  Sometimes, it's not a problem and doesn't matter much.  But, in complex pieces (IMO) it matters a great deal if you're creating conforming items.  There's only so much control you can have and it works best if you build you mesh to take advantage of that as efficiently as possible.