Oftentimes, it's really difficult to tell if your cat is sick. They tend to hide their illness until they are quite sick. So if in doubt, the best thing to do is get your cat in to your veterinarian ASAP. Here's some specific instances where you're better off being safe than sorry.
If you cat is open mouth breathing, has increased respiratory effort and rate(breathing with the abdomen, respiratory rate >50), has purplish blue or pale colored gums or tongue, is holding his head up high as if gasping for breath, or making wheezing/crackling sounds, you need to get to an emergency hospital ASAP. Do not tarry. Regardless of whether your pet is experiencing heart failure, an asthma attack, or has thoracic effusion (fluid around the lungs), your pet is in dire need of some oxygen and critical care!
Many toxin exposures need to be addressed within the first hour, ideally, before absorption occurs. You can always call a PET POISON HOTLINE (below) to see if what they got into is toxic. Do not try to make your cat vomit! Common toxins include but are not limited to sugar free gum, chocolate, rat poisons, slug bait, medications, grapes/raisins, certain plants, and household cleaners.
PET POISON HELPLINE 800-213-6680
ASPCA ANIMAL POISON CONTROL 888-426-4435
ANIMAL POISON HOTLINE 888-232-8870
If your cat has a seizure that lasts longer than 5 minutes it needs to be taken to a veterinary hospital immediately. There they may give your pet anti seizure medications and likely monitor for further episodes. If your pet has a seizure and seems to come out of it quickly as is acting relatively normal afterwards, you can probably wait and see your vet the next day to investigate possible causes. However, anytime your pet is suddenly acting strange neurologically ie, unable to stand, has nystagmus (eyes darting back and forth), is staggering or falling over, you are better off seeking immediate medical attention.
BLOATED OR PAINFUL BELLY OR NOT EATING
If your cat vomits just once or has a little diarrhea, you are probably ok waiting until the next day to have it seen by your regular vet. However, if the vomiting is occurring frequently (more than 2-3 times) without stopping, the abdomen is painful to the touch you need to get them in right away. You pet may have eaten something that has caused an obstruction or has a potentially serious condition called pancreatitis. Finally, if your cat hasn't eaten in more than a couple of days and is acting listless it needs to be seen by a vet very soon. Cat’s can actually go into liver failure quickly if they aren’t eating!
INABILITY TO URINATE
If your male cat is going into the litter box multiple times and seems like he's straining without much production of urine, he is licking at his penis, or is acting painful/cranky (especially in his back end) he may be obstructed. If so, immediate urinary catheterization and supportive care may be the difference between life and death. DO NOT WAIT!
DIFFICULTIES WITH LABOR
If your female is at term and labor has begun, DO NOT let her have active contractions for longer that 30-60 minutes without producing a newborn. Additionally, DO NOT let her go longer than 4 hours between kittens, especially if she is straining actively. She may have a newborn stuck in the birth canal. If you are unsure, call your local clinic, and they will help you determine if it sounds like your pet may require a C-section, manual extraction of the newborn, or if it's ok to just watch her at home for a bit longer.
EYE OUT OF SOCKET OR RED/INFLAMED EYE OR SWOLLEN EYEBALL
Most likely if you come home and your cat's eye is hanging out of the socket, you would not hesitate to take it to the nearest veterinary clinic. And that a good call because the longer you wait, the less chance of saving the eye and your pet's vision. But other eye issues are less straightforward. If your pet's eye is red, swollen, they are squinting or at rubbing at their eye(s) you should get them to a vet promptly as they could have glaucoma (high ocular pressure) , uveitis (infection of the uvea or the colored part of the eye), or a corneal ulcer, all of which can be extremely painful and affect vision long term if not addressed in a timely fashion.
No matter whether Scooter was just hit by a car, has fallen, attacked by a dog or other cat, was shot, or involved in any traumatic incident, he needs to be seen right away. Even though your cat might look stable, the extent of injuries may be hidden, they could be severely injured. Shock has a way of obscuring the seriousness of injuries. Let a vet do a thorough head to toe exam to determine if any treatment is needed. In this case, it's always better to be safe than sorry.
PARALYSIS OR INABILITY TO WALK
Sometimes cats can seem suddenly paralyzed, especially in the rear legs out of the blue. This can be the result of a slipped disc in the back, at blood clot, or a tumor compressing the spinal cord. Timely assessment and treatment may be the difference between your cat walking or not in the future. Cats usually present with legs or feet that are hard and cold to the touch. That can be the result of a clot (from untreated heart disease) which has lodged in the veins that supply their rear legs with blood. The condition is very painful and they are often crying loudly. The sooner you get your pet seen and it's pain relieved, the better.
EXTREME WEAKNESS OR LETHARGY
If your cat just seems really tired, weak, doesn't want to get up, and just wants to be left alone, there is probably something very wrong going on. It could be bleeding somewhere, having kidney or liver issues or a life threatening infection. Whatever it is, you need to have them assessed by a vet ASAP.
In all cases, use your intuition to help guide you in determining if your cat needs attention sooner rather than later. If things just don't seem right, they probably aren't. Call your vet. They can give you a pretty good idea what the urgency of the situation is. And if your vet's clinic isn't open, call your local emergency hospital. Don't put yourself in the position of feeling awful because you ignored symptoms that resulted in serious illness or, worse, death of your cat. In many cases, trying to save a little money or watching and waiting might become one of your biggest regrets. Your beloved cat, like a child, relies on you to take care of it. After all, it is a valued member of the family.