Defintion Diabetes Mellitus:
A condition characterized by Hyperglycaemia resulting from the body's inability to use blood glucose for energy. In insulin-dependent (type 1) diabetes, the pancreas no longer makes insulin and therefore blood glucose cannot enter the cells to be used for energy. In non-insulin dependent (type 2) diabetes, either the pancreas does not make enough insulin or the body is unable to use insulin correctly.

Symptoms:
1. Eating too much 
Due to lack of insulin, body cells can not absorb and utilize blood sugar and therefore the body is ultimately starving

2. Losing weight
Inspite of increased appetite, since the body continues to starve due to being unable to utilize the blood sugar

3. Frequent Urination
Urine glucose acts as an osmotic diuretic - drawing more water than usual from the body into the urine.  This therefore increase urine volume and the need to urinate

4.  Drinking Alot
Your pet is excessively thirsty due to the fact that more than normal amounts of water is being pullout out of the body and lost in urine

Diagnosis:
Your veterinarian may be suspicious of diabetes based on your observations, as well as general symptoms and a physical exam.  For a definitive diagnosis, blood and urine tests will be required in order to see evidence of high blood glucose levels and the presence of urine glucose.

In cats, stress can sometimes elevate blood glucose levels signifigantly therefore it may be also necessary to measure fructosamines (a potortion of blood glucose that is bound to a blood protein called albumin) to confirm the diagnosis

Please refer to these additional sites:
How to do a Ear Prick to Test Blood Glucose
Blood Glucose Testing Website - A Website to Help Diabetic Pets