The timing chain stretch can be checked using a dial indicator on one of the valves in the number one cylinder. You will have to get exact procedure from a factory manual. It is not difficult if you have the procedure and the dial indicator.

Timing chain stretch rate is closely related to oil change frequency. If the oil has not been thoroughly and religiously changed, the particulate matter in the dirty oil will wear the chain rapidly.

I have never had a chain failure in a 616 or 617 motor, but I do know that, if oil changes are neglected they can fail.

The chain is so easy to replace, if you're worried, roll a new one in. It's probably the second cheapest engine insurance you can buy behind frequent oil changes.

Good luck,