Oh yes, they know, they understand, and they take evasive action against all humans.
My deaf, blind and walking-around-in-circles-due-to-a-stroke cat overheard a phone conversation with the vet arranging to put him to sleep. He heard, understood, and promptly dragged himself off the sofa and went out in the garden to sniff the flowers (with his one remaining functioning sense) and frollick in the long grass, as a way of saying "look at me mum, I've still got quality of life, please don't kill me". Bastard. Of course I couldn't do it then!
I should have put him in the freezer.