I'm more of a hobby bicycle mechanic. Isn't there some similar tool for the meassuring of the chain as you get for bicycles? Just push it down and you directly see if it's time to change the chain. If not done early enough you have to change all the sprockets too because of wear. If done in time, you might not have to change all the sprockets.
You can measure it with an ordinary caliper, but the problem is getting access. The timing chain needs to be continuously supplied with oil, so it is in rather than on the engine and can rarely be accessed without pulling the engine from the car. It takes several hours just to access the chain for inspection, so it's not something you'd periodically check - if you suspect a timing chain problem, you might as well replace it.
