There’s a tension and maybe responsiveness to skin and muscles that is uncanny when missing. Not sure many here could 100% recognize that very early on at the point of death, but at some point there is a wariness/unnatural look to the skin. Between that and our assumed ability to pick up on a complete lack of movement/breathing/pallor makes it reasonably certain that there is a “something” we recognize as missing, even if it’s hard to describe perfectly.