Cats have evolved to try and conceal when they are sick. So you need to pay more attention and watch for the signs below and call your vet if you’re not sure.
Hiding, decreased energy, or other behavioural changes
Bad breath in your cat.
Changes in litter box usage (going outside the box, straining, etc)
Increase or decrease in appetite or drinking
Sitting hunched over
Obvious illness or injury: vomiting, diarrhea, coughing, sneezing, wounds, limping etc.
.