Viet Thanh Nguyen on page 267 of The Sympathizer:

You know how to tell if someone’s really dead? Press your finger on his eyeball. If he’s alive, he’ll move. If he’s dead, he won’t.

When I was an EMT, a paramedic told me of another way to tell if someone’s playing dead: rub the knuckles of your first two fingers hard against their chest. No one can pretend after that.