Personally I inherited a mitochondria disease (LHON), so I'll be able to tell if mine becomes bad because I'll go blind.

Can't recommend you do this though.

I do group fitness classes, like CrossFit, body combat, spin class. When you have a room full of people all doing the same thing you can quickly tell which things you are above or below average at. Not very scientific perhaps but this is my current strategy.

For me I am below average at endurance, so that is where I assume the mitochondria come in.

That's the clever bit - you extract them from pigeons instead!