I'm not aware of anything Ollie, as far as I know the only indicatior (if it looks OK) is if it doesn't have the desired effect i.e. your levels climb/stay high after injecting.
You'll know when you get unexplained highs despite feeling well and trying your best to correct, it's happened on a couple of occasions to me mainly due to the insulin being to warm beforehand.
Somewhat longer answers lead to the thought that "Designing a stable monoclonal antibody that discriminates would be tough, but commercializable." If anyone's an entrepreneur or a biochemist here, perhaps you can do something with this info!
This is fascinating and if a test could could be made reliable and cheaply enough, could save the NHS a lot of money in the long run. It seems to me that something a bit like a test strip or ketone stick would do the job nicely if you could find the right reagent. Anyone have a tame research chemist who wants the Nobel Prize for Diabetes Care?
Does anyone know what the normal Ph of insulin is, or what chemical change would need to be measured to decide if the stuff had gone off?