So, I think I answered some of my own questions.
Chain stretch – 3 ways of measuring or at least identifying;
– I used a straight metal ruler. Put a new chain against it. End to end, the new 219 chain fits exactly in the center of 2 pins. Next put the suspect chain against the ruler. If the chain pins overhang the ruler, it is stretched.
– I also put the new ‘benchmark’ chain directly against the suspect chain to verify that it was stretched.
– I then wrapped the chains, one at a time, around an entire new rear sprocket. and held the excess chain tight against the sprocket with with one hand. A new chain could not be lifted off the new sprocket and fit tightly, the used chain could be lifted off the teeth of the sprocket and had play.
Now to figure out how to tell if a sprocket is still good.