The best investigation into insulin resistance that I have yet found is by Jason Fung in his book The Obesity Code. Most of the same info is freely available on his website here:
https://intensivedietarymanagement.com/
He is a nephrologist (kidneys) who runs a diabetes clinic, because most of his nephrology patients are diabetics.
You are far better going to him for info on insulin resistance (because he provides all the references!

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But I can give you a VERY brief explanation, from my limited understanding...
- humans have a great coping mechanism that means we can become resistant to things, which increases our tolerance of them.
- this resistance builds up to food (if they include toxins), drugs, and even our own hormones, if they are produced too much and too often.
- when T2s are described as 'insulin resistant' it means that we have been producing insulin in so much quantity that the cells of our bodies become resistant to it.
- so our pancreas produces MORE insulin, and our cells become MORE insulin resistant
- sometimes T2s are producing MASSES of insulin, but their cells are resistantly ignoring it (think of someone constantly turning up the volume of a stereo - the insulin - while everyone at the party just shouts louder and louder, to try and talk to their friends - the resistance.
- once we become resistant, our body is flooded with insulin
- insulin is the hormone that shoves glucose into our cells where it is stored as fat.
- being fat increases insulin resistance
- insulin and insulin resistance are also a known trigger for hunger
It becomes a vicious cycle of carbs-glucose-insulin-insulin resistance-fat deposits-hunger-carbs
The answer is to get off the cycle, reduce carbs, reduce insulin, and the insulin resistance drops. Other ways of reducing insulin resistance (temporarily) are exercise and fasting, but eating carbs again will cause the insulin resistance to rise again - in T2s. Metformin reduces insulin resistance a little. Please note that insulin resistance is possible for anyone, including T1s and non diabetics, but
I think the problem is always present for T2s, unless they control it with diet, exercise and/or fasting. Insulin resistance is present before we become pre-diabetic, then diabetic, and before we start to gain weight. It is the cause, not the symptom.
The usual test methods used in the UK test for glucose levels, not insulin levels.
I believe there is a test used elsewhere called the Insulin Clamping test, which measures the amount of insulin released after the ingestion of a set amount of carbs, but I don't know the details, since there is more chance of England winning the Euros than there is of me ever being insulin clamped.