Usually, a blood work will show you how your dog or cat's kidney health is doing. Vets look at the BUN, Creatinine (waste indicators) or SDMA to figure out how your pet's kidneys are doing.  A urinalysis is needed to help confirm kidney disease in dogs and cats. Make sure your dog or cat is staged for kidney disease by the vet meaning they tell you what Stage the disease is at and what that means. 

Typically, CKD is staged between 1 to 4 with the latter being more serious. Each stage can be thought of as how much filter function is lost or remains. 

Yes. There are different approaches depending on which vet you talk to. 

Typically, traditional vets have recommended lowering protein in order to lower phosphorus but that has been found by some studies to be an erroneous suggestion.  e.g.  A study by Dr. Delmar Finco, showed that “mortality was actually higher in the low protein group” for dogs with renal disease where “there were no adverse effects of the high protein diet.” [Effects of Dietary Protein Intake on Renal Functions” 1992].
Holistic vets would suggest high-quality protein and lower phosphorus by choosing appropriate proteins e.g. skip fish. They would also recommend no kibble at all and focusing on fresh. 

Depending on where you are staged e.g. 1 to 4, we normally do not recommend the kibble renal diets instead leaning toward the holistic side. Here are some quick tips for managing kidney disease in the early stages:

1. Switch from a dry kibble or diet to one with lots of moisture e.g., canned pet food or frozen raw/ cooked food.
2. Provide your pet with adequate amounts of water and if you are using a freeze-dried or dehydrated, make sure to add water to the food. 
3. Split meals into smaller ones and more frequency e.g., 3 instead of 2 meals puts less stress on the kidneys. 
4. Add omega oils to your pet food. 
5. Evaluate your food for lower sodium and potassium. 

For later Stage CKD, go with foods with low phosphorus content.