Our pets can't tell us when they are not feeling well and we may not always be able to tell if something is up unless they are showing obvious signs. One good and easy way to tell if there is cause for concern in regards to your pet's health is by their poop. Their poop can tell you a lot!
It's easy to tell if something is going on with indoor cats because they use a litter box that you scoop daily. It can be challenging to narrow it down to which cat it is in a multiple cat household so knowing each cat's habits and observation are key. (This is also one of the reasons litter boxes should be scooped daily so if something does show up that warrants attention, you can get it checked out asap because the sooner you notice it and get it taken care of, the better.) It's more difficult with outside cats or cats that go primarily outdoors unless you observe them or can find their poop.
Our cats are indoor/outdoor. We have litter boxes inside and we also made an outdoor litter box in our backyard made out of sand for a child's sandbox with a layer of rocks on the bottom and landscape timbers for a frame. Our three-legged cat Kilo uses it all the time. He has a hard time digging in the backyard with only one front leg and finds it easier to dig in the soft sand.
When it comes to dogs, unless you pick up their poop regularly and/or take your dog for a daily walk you may not notice that something is off by way of poop detection. I have discovered worms and other serious issues going on many times over the years while pet sitting or walking dogs and the dog's guardian had no idea. It's important to watch your pets and know what their elimination habits are.
I see a lot of poop and I've seen every kind shown in the video below! Unfortunately, one very common type I see when it comes to cats in particular, is very dry, hard stools. This is an obvious outward sign of dehydration. This is due to a dry food diet. Cats are not physiologically made to sustain vibrant health on a diet of highly processed dry kibble and it shows in their stools among other things.
Though this informative video shows a dog, the same holds true for cats as well.