If you are worried that your cat might have diabetes, here are two steps that you should take.
Take them to a vet clinic to have them examined by a vet.
If you have even the smallest suspicion that your cat might be diabetic, it is absolutely critical to have your pet examined at one of your local vet clinics as soon as you can. The reason for this is as follows: if your cat does, in fact, have diabetes, and it remains untreated for an extended period of time because you decided to wait to take them to the vet, they could develop serious complications as a result of this disease. Untreated feline diabetes can, for example, cause cataracts, renal disease and permanent nerve damage. As such you must act quickly in this situation.
When you go to the clinic, the vet will take a sample of your cat's blood and urine to check their glucose levels. If the results of their tests confirm that your cat has diabetes, they will provide you with very specific dietary instructions, as well as medication that your cat will probably have to take daily for the rest of their life.
Encourage your cat to exercise if the vet tells you that they are overweight.
If the vet informs you that your cat is overweight, it is extremely important to not only follow the dietary guidelines they give you but to also encourage your cat to exercise. The reason for this is that excess body fat in cats is thought to increase insulin resistance (and thus make these animals more susceptible to diabetes).
In combination with a calorically-restricted diet, regular exercise could help to reduce your cat's body weight and make them more insulin sensitive. This, in turn, could minimise their chances of developing the above-mentioned health complications and perhaps even lead to them requiring a lower dosage of their medication.
Whilst encouraging a cat (particularly an indoor one) to exercise is no easy feat, there are ways to do this. You could, for example, buy a small laser pointer and spend about 20 minutes or so each day moving this around a room, encouraging your cat to chase the laser beam (this is something that a lot of cats enjoy doing). If you don't want to use a laser pointer, you could buy a ball of twine or knitting wool instead, and trail the string along the ground and encourage your cat to chase it.