I don't think there's a general solution for it, if you have the cache properly encapsulated. I've struggled with this problem before, and I haven't found an answer that has satisfied me.

But you may be able to write some tests tailored to how you've implemented the cache for your specific problem. If you're caching database or network results, you might be able to trace the traffic over the database or network handles and conclude a query not repeated. Or you may be able to gain some insight using tied variables, or overloaded objects. But they are not general solutions, and will only work in some cases.