I'd second what HoneyFern said, I can usually tell by feeling the cats ears as they should be sort of cold-ish (depending where they have been) if they are warm to the touch then that's usually a sign that they have a tempreature.

Just try and monitor the water intake aswell if you can and see whether the cats passing motion in the tray (if he is an outside cat I know it's difficult to do so.)

Hope that helped, if you are still worried and she's not 'better' in a couple of days and still off her food check with the vet.

Edit: Actually, is her tummy 'hard' at all near where the genitals would be? Sometimes cystitus can affect them like this I believe

__________________
*Katania aka Shin*

Mother to the best cats:
Sooty (5yrs)
Smokey (4.5yrs)
Bandit (3yrs)