I like to compare the chain coming off to the chain I am putting on. You can visualize stretching by laying them on a clean surface side by side. Also, hanging the used chain and observing lateral slack.
I recently had a 86 VFR that had the chain tensioners installed inside out. When I tried to reassemble the same way, I simply could not make it fit. When I inspected closer, the wear indicator stickers were facing the inside. I turned them around and the rear wheel slipped right into place. However the chain quickly weared after this, and is being replaced. I am betting that chain will have a considerable amount of lateral slack.
04 KTM EXC250
74 CB350F in process