Physiological insulin resistance (PIR) only happens in the absence of glucose from carbs as it is a mechanism the body uses to preserve available glucose for the brain and some red blood cells. You will know if you have it by using a blood glucose meter on fasting and before meals, and bedtime. If you notice these levels rising, with a trend line on the way up, you may have PIR IF you have been eating a true keto diet. You may not - it just may be you are crossing the line to diabetes, but it is the only indicator we have. You can ask for insulin tests from the GP but will probably be very lucky to get them.
PIR is often only temporary and can be stopped by adding in a few extra carbs for a few days. Yes, you probably would fail an OGTT but you would also be likely to fail one eating a non-keto but low carb diet. This is why the instructions to prepare for an OGTT is to eat around 130-150g carbs a day for 3 days.