I know there is a glucose fasting test whereby your blood is tested and then you drink a glass of glucose water and test again. However I did not know there was a test for measuring the level of glucose tolerance. But I've never heard of a test for actually measuring insulin resistance.
There is a test (sorry forgotten its fullname) for measuring the amount of insulin produced. Kraft used it for 30 years, and called it an Insulin Assay, I think. Not used in the UK to my knowledge. The key thing would be how much insulin was needed to deal with the glucose in (for example) an oral glucose tolerance test. If a 'normal' produces x amount, to keep blood glucose 'normal' during test. and a 'insulin resistant person' used x + extra insulin to keep bg 'normal' then that person has a measurable amount of insulin resistance. The key thing is that their insulin resistance will not show up as glucose intolerance and raised bgs, because they are pumping out enough insulin to keep bg 'normal' with the knock on effect that it causes insulin resistance. You may find this video interesting.
C-peptide? It actually measures peptide in the blood which is meant to be equal to the amount of insulin. Both are produced in the pancreas at the same rate.
The test used for clinical studies measuring insulin resistance is called a "glucose clamp": https://en.wikipedia.org/wiki/Glucose_clamp_technique There is also another method that is used to approximate this outside of clinical studies called a HOMA calculation. It uses fasting insulin and fasting glucose: https://www.dtu.ox.ac.uk/homacalculator/ I think a lot of us would be really pleased if someone came up with a way of measuring insulin via a meter in the same way we can already do for glucose
Thank you @Indy - Glucose clamp was what I couldn't remember. @DavidGrahamJones the c-peptide does measure how much insulin you produce, but that could also be in response to a high carb diet, rather than a normal carb diet with high insulin resistance, if you see what I mean. You need simultaneous bg testing and insulin (glucose clamping) to see their relationship and discover the insulin resistance.