If your car starts easily and runs well, has no changes in fuel economy, and you have been keeping up with basic maintenance, it's probably fine.

Same with people. Sure there are some people who just drop dead out of the blue. But most people have symptoms. Unexplained weight gain or loss. Fatigue. Difficulty sleeping. Changes in bodily functions. For most people, if you have no symptoms and feel healthy, you most likely are. If you have family history of certain problems, of course be more vigilant about that.

We can't possibly continually test everybody for everything that might possibly be wrong with them, at least not until we have Star-Trek style medical devices. Unjustified testing in the absence of symptoms will simply further drive up the cost of medical care for those who really need it. One could argue that it already has.