While IF does improve insulin sensitivity, it has other touted benefits like improved lifespan, enhanced immune system, and improved resistance to neurological disorders like parkisons and alzheimers via upregualtion of brain-derived neurotrophic factor. However, improved insulin sensitivity and lowered levels of inflamatory factors like homocysteine are the only things that I have seen shown in humans thus far. I havn't checked on any new research in a couple months though.
Tangible signs your insulin sensitivity is improving:
You can go longer without feeling hungry.
Crave sugary/sweet food less.