How do I assess whether a change in my cat's behavior means they're sick?

Well, certainly, if they're not eating or not drinking, if they're vomiting, having diarrhea, or if they're not urinating or not pooping, those would indicate a medical problem. If the cat is not interacting with you the way they usually do or sleeping all the time, those would be the things you would be looking for to indicate a problem.

A change in your cat's behavior can oftentimes indicate something is amiss. Keep in mind that the seasons change and all those, but let's say for example your cat's hiding and then crying. You've probably got an underlying litter box issue that you don't even know about and the cat is manifesting his issue with that that way. The reason that that happens is because if a kitty doesn't want to go to the litter box, he or she is not going to use the litter box, which is either going to result in them A. peeing on your dirty laundry or your bed or in your closet or somewhere where it's at least relatively clean. Or they're gonna simply hold it until they can't take it anymore. That's how bacterial infections happen.

That's really good, and I'm not even sure I could give a ready-pet answer for that. If you notice a change, and you know your kitty better than anybody else does, that's the important thing. If you notice a change that has you concerned, you asking the question is concerning enough. I would recommend you seek out your veterinarian to get further advice and investigation.