An A1C test will only show problems after you've gone through a cycle of your body resisting insulin and your pancreas being overworked (resting insulin 4-7x normal) and then your pancreas wears out (damaged beta cell production) and then your resting glucose rises. This process can take many years. A resting insulin test will show resting insulin rising years before an A1C test shows a problem.

A1C is not the fasting glucose test.

It's not, the same stands... you'll go through years of elevated insulin before you see fasting glucose or A1C rise. A glucose tolerance test could be useful too, but a bit of a pain compared to simply testing insulin levels and comparing to the prior year.