I want to clarify something here for those that are following along at home. A random blood sugar test can be used to indicate diabetes, but just because your blood sugar is less than 200 mg/dL, does not mean that you don't have diabetes.
An analogy would be this: if you weigh over 600 pounds, you are definitely obese. Just because you weigh less than 600 pounds does not mean you are not obese.
I'm diabetic, but my blood sugar was never over 200 mg/dL any time I checked.
As I mentioned, it's much more common to use A1C and fasting blood glucose tests to diagnose diabetes. Fasting is good because blood sugar fluctuates throughout the day. It should be at its lowest point after an 8 to 10 hour fast, so for most people, this is first thing in the morning. The other common test most diabetics are asked to do is "postprandial". This means after a meal. Now timing after that meal is subject to some debate, but typically 2 hours after eating is used as the "standard" for diabetics. For everyone, even non-diabetics, your blood sugar will start going up roughly 10 minutes after starting to eat. While this is heavily dependent on what and how much you ate, in a non-diabetic, your postprandial blood sugar should peak within about an hour of eating and should be back to normal in 2 to 3 hours.
For diabetics, this normally does not happen as fast, so the peak blood sugar concentration is typically closer to 2 hours after eating. In short, most type 2 diabetics are asked to take a postprandial reading 2 hours after eating.
Both fasting and postprandial readings are used to determine how well the disease is managed. For example, a high fasting blood glucose test might indicate that your diabetes medication needs to be adjusted. Another example might be if the post prandial reading is actually lower than the fasting number. If this is a consistent trend, it might indicate reactive hypoglycemia.