Based on my personal experience with Spooky(GA) and what I have seen/read here for over 10 years (gosh, how did that happen?!) insulin resistance is frequently caused by excessive amounts of insulin.
One of the best tests for resistance is to cut the dose in half and monitor for a couple of days. In most resistance cases, the reduced dose will actually reduce the sugar levels in the blood.
I would not say the best test for resistance is to cut the dose. I think you could be asking for trouble taking that route.
I do agree with asking the important questions like home testing and of course diet. There have been several cats who end up diet controlled, and after high doses, so home testing first, then adjusting diet, must be handled first. You will then have a better picture of what's going on. I think it would be a relief to the new person if told that hey let's just get you testing and then off the high carb foods because there have been tons of cats who are diet controlled and never even need insulin! It's excellent to try before starting insulin, but more tricky if insulin already started. It can be done though.
Once the testing and food issues are put in place, you can then look to adjusting the dose.
I was driven to try the 'drop back to 1u BID from 4u BID, he's getting too much insulin' when I adopted Ollie, and that approach was painful to see. Even with a proper diet and extreme home testing showing all 400s, still I was told that I was overdosing him. I can tell you first hand that such an approach hurts the cat and the owner.
I was not new to feline diabetes as Shadoe had just been formally dx as having acromegaly. She had absolutely zero physical features - she was small, petite, fine featured face, small paws and no belly - and had not been on insulin long - she began on Caninsulin Oct/09 but quickly switched to Lantus Nov/09 on finding this board. I was helped by great people on this site - jojo, Carolyn/Spot, and Lorna/GC as well as others. I followed the protocol to the letter and her dose kept rising. We pretty much knew testing would be positive for Acro and it was, in March/10. She was not on too high a dose and built a resistance.
When I adopted Oliver, he had been on insulin barely a month, since Feb08/10. He arrived mid-March17/10 and tested positive for both acromegaly and IAA April11/10. Surely not enough time there to build up a resistance to reach over 300 for acro and 60% for IAA in 2 months.
I would say that the resistance comes before and causes the diabetes because there is nothing wrong with the acro cat's pancreas. The tumor hormone secretion is the issue with insulin resistant cats.
It is important to keep in mind that there are some cats who really do need a higher dose and a perfect example is Randi/Max. Max and Shadoe were neck to neck, both with doses rising. Max got up to 6u or 6.75u I think and then he broke through, I guess pancreas was healed and able to take over. And now he is OTJ. You could suspect maybe he had some IAA and fought it back but no, his numbers did not support that theory. Max just needed a bit more insulin and when he was able, he came off insulin.
I think a few prelim questions about type of insulin and diet, plus how the dose was reached are a perfect start, along with home testing. A weekly post or sticky would be a great idea.