The article doesn't mention this, but it is mentioned in several of the low carb books that I have read; if you tend to accumulate fat in your belly instead of your hips, butt and thighs (the infamous beer gut or apple shape), it's a good sign of insulin resistance regardless of how overweight you are.
Insulin resistance is also the precurser to diabetes; if you have it and don't do something about it, you will eventually develop diabetes (more likely sooner than later).
Of course, the only scientific absolute way to determine insulin resistance is to have your circulating blood insulin levels checked, but often a diagnosis is made based on presenting clinical symptoms.