The accepted ways of screening for diabetes are a fasting
glucose, a random
glucose(not as good) or a 2 hour
glucose tolerance test. A HgB A1C is sometimes used by doctors but it is not widely accepted as a good screening measure, so I suggest a fasting or an OGTT.
You should also have a thyroid screen(TSH) done.
Fasting glucose is not perfect, so yes you can have a "false negative" in terms of saying you have no abnormalities in glucose handling.