[atleta]

Well, they could measure the biological ages of people who die (or maybe who have terminal illness) and compare it to people who continue to live on.

If they can get the price of the measurement low enough (which Sinclair claims to happen soon), then we can just wait and see: by doing a large number of measurements we'll have people who end up dying pretty quickly. (Yet another way to do it would be just to collect a large number of samples and measure those who die and compare it with some who do not, during the experiment.)