I've seen websites that say anything over 10 implies insulin resistance, but other sites say it is very difficult to diagnose. If you are IR, you will likely have physical symptoms of IR, and you may have physical symptoms without bloodwork showing that you are IR. My fasting insulin was 13 - not too bad, but I had many symptoms. Acanthosis Nigracans, oily skin, hair thinning at the temples, hypoglycemic episodes, and feeling hungry all the time.
This website talks about testing for IR:
http://www.wdxcyber.com/ngen10.htm