It is not easy to tell whether a cat is not drinking enough. There are, however, a few indicators that may help you: Basically, a cat's water requirements depend on its age, weight and size, its food, the room temperature, its level of physical exertion and any illnesses. Therefore, only an average value can be given, which is 55 millilitres per day and kilogramme (body weight). A cat weighing five kilogrammes therefore needs around 275 millilitres of water. If it eats around 250 grams of wet food a day, it will already consume 200 millilitres of water (assuming the food has an 80 percent moisture content). Therefore, the cat only needs to drink an additional 75 ml of water. If, on the other hand, it is only given dry food, the amount required is significantly higher.
If you are not sure whether your cat is drinking enough, you can measure the amount of water provided and check the remaining amount with a measuring cup 24 hours later.