According to the American Diabetes Association, the symptoms of Type 1 diabetes are:

Type 1 Diabetes

* Frequent urination
* Unusual thirst
* Extreme hunger
* Unusual weight loss
* Extreme fatigue and Irritability

And Type 2 Diabetes:

* Any of the type 1 symptoms
* Frequent infections
* Blurred vision
* Cuts/bruises that are slow to heal
* Tingling/numbness in the hands/feet
* Recurring skin, gum, or bladder infections

However, not all symptoms have to be present to indicate someone has diabetes. In fact, people with Type 2 diabetes often experience no symptoms at all.

Source: http://www.diabetes.org/diabetes-basics/symptoms/

As far as prevention, type 1 can't be prevented. This type of diabetes occurs in only about 5% of the people who have diabetes and is usually diagnosed during childhood or early adulthood.

Type 2 diabetes does have a lot to do with lifestyle. To help prevent or delay the onset of type 2 diabetes, maintain a healthy weight. Eat a well-balanced diet and exercise regularly. And don't smoke. People with low HDL cholesterol, high triglycerides, or high blood pressure run a higher risk of developing Type 2 diabetes than those with normal levels, so keep an eye on these.

Source: http://www.diabetes.org/diabetes-basics/prevention/

The most common symptoms of diabetes are increased thirst and frequent urination. If you're experiencing these symptoms, you should definitely consult your doctor right away. There are other symptoms, too, including increased hunger, dry mouth and unexplained weight loss.

Source: http://www.simple-diabetes-notes.com/diabetes-warning-signs.html

Diabetes mellitus is a medical condition that affects the metabolic processes in our body especially the regulation of blood sugar. Many people are affected by this disorder and many are still unaware that they already have the disease. That is why it is very important to recognize the signs and symptoms of diabetes so that proper and prompt treatment can be administered.

The signs and symptoms of Type 1 and Type 2 are almost similar. In Type 1 diabetes, the high glucose level is due to lack of insulin because the insulin producing cells have been destroyed. In Type 2 diabetes, the high glucose level is due to the body’s cells become resistant to insulin being produced by the body. Any alteration in the regulation of blood sugar level will signal the body with signs and symptoms. With lifestyle modifications and proper medical attention, these signs and symptoms can be managed.

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Some of the common 'early warning' signs of diabetes are:

Excessive thirst
The first symptom of diabetes is often excessive thirst that is unrelated to exercise, hot weather, or short-term illness.

Excessive hunger
You are still hungry all the time even though you've eaten.

Frequent urination
Frequent urination is often noticed because you must wake up repeatedly during the night.

Fatigue
Tiredness and fatigue, possibly severe enough to make you fall asleep unexpectedly after meals, is one of the most common symptoms of diabetes.

Sudden weight loss
Rapid and/or sudden weight loss (any dramatic change in weight is a sign to visit a doctor)

Minor, less recognizable symptoms

Often the 'minor' symptoms of diabetes go unrecognized, and physical and neurological problems may arise.

Blurry vision
Blurred vision may occur because diabetes can lead to macular degeneration and eventual blindness.

Numbness
Numbness and/or tingling in the hands and feet may occur due to peripheral neuropathy, a symptom of diabetes, causes nerve damage in the extremities)

Slow-healing wounds
Slow healing of minor scratches and wounds may be the result of diabetes-related impaired immune system function.

Recurrent yeast infections
Recurrent or hard-to-treat yeast infections in women are another sign of impaired immune function.

Dry skin
Dry or itchy skin may result from peripheral neuropathy which affects circulation and proper sweat gland function.
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Source: http://www.informationaboutdiabetes.com/symptoms-of-diabetes