There are several signs of poor nutrition that you can look for in your Siamese cat, including:

1. Loss of muscle mass or visible ribs: If your cat’s muscles are wasting away or you can see their ribs, it may be a sign that they are not getting enough protein or overall calories in their diet.
2. Dry, dull, or flaky coat: A healthy cat’s coat should be shiny and smooth. If your cat’s coat is dry or flaky, it may be a sign of a deficiency in certain essential fatty acids or vitamins.
3. Slow wound healing: If your cat is not getting enough protein, they may take longer to heal from injuries or wounds.
4. Constipation or diarrhea: If your cat is not getting enough fiber in their diet, they may experience constipation or diarrhea.
5. Obesity or weight gain: Obesity can be caused by consuming too many calories, not getting enough exercise, or a combination of both. Make sure you feed them appropriate portion sizes, and check their body condition score.
6. Fatigue or lethargy: If your cat is not getting enough nutrients, they may feel tired or lack energy.

It’s important to note that these signs can also be caused by other factors, not just poor nutrition, so it is always best to take your cat to a veterinarian for proper diagnosis and guidance.

Also, consult with a veterinarian or a feline-nutrition expert to select a diet that is nutritionally complete and appropriate for your cat’s specific needs and life stage.