Answers:
because you can conduct tests, and manipulate varriables to show that they died instantly.
I mean, thats like saying if I were to decapitate someone, in one drop of the blade, how do you know that it didn't just kill them instantly.
If he was a live for a second or two that would have been said, commonly followed by those seconds must have seemed like hours/days.
This is like saying how do we know that when someone has their head cut off they are dead. Maybe the body just tricks us and stops its pulse and just lives off the oxygen that is in the blood while it sits at the limbs.
This is how they can be sure though, and that is that it has been tested. If he was in a car crash, then the actions of the crash, like hitting into the guy, and flipping around in the air a few times he would have felt and been alive for until he hit the tree.
Sorry if this was a little graphic...But that is how it goes.