[header] How to know if your cat is sick [title] Pay attention to how much your cat sleeps. [step] Sick cats will sleep more. If your cat doesn't have other signs of illness, such as vomiting, diarrhea, loss of appetite, or obvious swellings, then keep him/her under observation. 
If symptoms develop, get him/her checked by a vet. [substeps] If your cat isn't showing other symptoms, monitor him/her for 24 hours (of course, it's fine to get her checked by a vet before this if you are worried).