What is the best indicator that you’re under absolutely NO risk of burning yourself out due to too much exercise? ~

I'm not sure there is one but at a guess
I'd say maybe someone who has excellent aerobic fitness and muscle tone would be the least likely to burn out, because they have enough experience to read their bodily symptoms and know their limits.
The alternative might be the opposite, someone who is exercise phobic and would never exercise enough to put themselves at risk.

Don't exercise.

- You're pushing up daisies.

- You get a voice operated remote control (excessive thumb exercise is the leading cause of exercise burnout among couch potatoes)

A really fat Belly.

Too old to cut the mustard anymore. 

You look forward to your daily run and weight set