It's very important that once you are over 40-50 (depending on your individual risks) you check with your provider about having an annual fasting glucose done. As you say, many people with diabetes don't know they have it because Type 2 diabetes often presents with few or no symptoms.
Risk factors for diabetes include:
-family history of Type 2 diabetes
-being overweight
-lack of exercise
-schizophrenia (due to the medications, not the disease itself)
-having had gestational diabetes or having had a baby weighing over 9 pounds at birth (could be undiagnosed gestational diabetes)
-history of polycystic ovary syndrome or other insulin resistance syndrome
Not every heavy person develops diabetes, and it's possible to develop diabetes even if you're not overweight. For example, between 5-10% of women have a disorder called polycystic ovary syndrome, or PCOS. PCOS is fundamentally an insulin resistance syndrome - your body needs more insulin than normal to lower your blood sugars. Approximately half of all women with PCOS are overweight or obese, and they often struggle greatly in trying to get the weight off, due to the insulin resistance. (Insulin makes your body store fat.) The other half of women with PCOS may be normal or even underweight, but they are still at risk for developing diabetes because of the insulin resistance.
Note: Some women may wonder if they have PCOS because they have a history of ovarian cysts. The answer is, not necessarily. It's possible to have ovarian cysts and not have PCOS, and it's possible to have PCOS and not have ovarian cysts. The cysts that are present in PCOS stem from the insulin resistance problem. Cysts can be caused by other issues that have nothing to do with insulin resistance, so if you're not sure, see your provider for blood testing.