The keepers know. The keepers realize that to the tigers they are a potential dinner. At the Zoo one does not go into the enclosure even with the nicest tiger. There, while the big cat is outside, they check out any untouched food, any abnormality in the feces. They get to know cats and the clues they leave.
Some cats are light eaters anyway.
Some are stoic about illness and have to be watched for small signs of pain. Between the enclosure and the yard are two steel doors with dead bolts that set off a runway. Keepers move the cats along this runway with the aid of sliding doors on pulleys, color-coded for safety. Thus, doors leading into the runway are blue with blue handles, and doors inside the runway, used when more than one tiger is being run through, are yellow. In the evenings tigers are brought back inside.