system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How a doctor knows when somebody 'died on impact'.

Aside from the obvious like decapitation or something like that.
gpt
Technically... they don't. But, it seems plausible and sounds much nicer than, 

"due to the injuries it's possible he died immediately, however it is also possible he spent a few minutes trying to move around slicing his muscles apart on broken bone fragments until he choked to death on his own blood. Goodnight, Timmy."