Usual signs are inactivity, not drinking or eating, lays around, appearance of the third eyelid. Best way to tell if your cat has a fever is take the rectal temp. Depending on the age of the cat the temp (normal) should be: 100- 102.5 degrees any higher get the cat to the vet NOW! Dehydration is a real danger.
It is not always wise to depend on the saying that a warm nose and warm ears mean fever. I had a cat with a temp of 105 once with the coolest nose of all!
Her whole body felt warm, her nose was cold but everywhere else warm. I have noticed that over the past few days, possibly a week now, she has been sleeping alot more than usual, and she has just been acting unlike herself. She has been a lot more "clingy". So I have been paying close attention to her because I was worried she was getting sick. The only other thing I noticed was that she has been feeling warmer than usual. Which made me think it was possibly a fever. I called the vet today and tried to explain, but she said she didn't seem to have any real signs of being sick. She did get spayed a month ago and the vets office said sometimes they go through changes after that. It's so hard to explain, I just know that she is not herself.... Ugh, I wish she could just talk to me!
Oh, I forgot to mention she has been eating, twice as much as she did before she got spayed. She also drinks alot of water.
Do you know how to take a cat's temp? Your best bet is to take the temp and if she has a fever get her in to be seen.
This thread talks about how to take the temp. If you have help to hold the cat, that will help you as well. But if not, the best place to take a cat's temp IMO is on top of your washing machine. The cat fits under your arm, and you can hold the thermometer safely it doesn't need to go in very far at all to register-