My father's last words were 'That was nice soup' before he quietly slumped over with a heart attack (nothing to do with my mother's cooking!). Whilst waiting for the ambulance she noticed the cat's eyes suddenly go as large as saucers and it's hair stand on end before backing out of the room. That's when she realised Dad had slipped away.
I've only witnessed the death of small squeaky or fluttery things rescued from the cat, but it's unmistakeable when the life leaves them. Like KT says, it's in the eyes, as if a switch has been turned off.
I'd accepted that you just 'know' when something is dead without giving it any thought, so I've found this thread interesting Lynne, death being such a taboo subject normally.