Ideally, the test conducted is an Insulin Tolerance Test (also known as Insulin Stress Test). While it may be inconvinient to go to a lab and stay around for 2 hours it is the test that will provide the most conclusive results.
As a 2nd best option one can measure with a single phlebotomy: insulin, glucose (which together will provide the HOMA-IR ratio), leptin & adiponectin. These markers are available from Doctor’s Data.
HbA1c, despite its popularity, is a poor indicator of diabetes. It measures the amount of blood sugar (glucose) attached to your hemoglobin. There are 2 pitfalls to using HbA1c:
1. The 1st is the assumption that everyone’s red blood cells (RBC) live equally long. In healthy individuals, RBC live longer while in unhealthy ones (such as those with diabetes or prediabetes) they live less. As healthy individuals’ RBC are alive for longer their HbA1c will tend to be higher, while those that are unhealthy will tend to show better results.
2. Ultimately diabetes is a problem of insulin and not sugar. Those that suffer from diabetes are administered insulin, not sugar pills. Unless you measure your insulin levels you are guessing.