Not at my workstation right now, but simply ask claude to analyze jsonl transcript of any session, there are two cache keys there, one is 5m, another 1h. Only 1h gets set. There are also some entries there that will tell you if request was a cache hit or miss, or if cache rewrite happened. I've had claude test another claude and on max 5x subscription, cache miss only happened if message was sent after 1h, or if session was resumed using /resume or --resume (this is a bug that exists since January - all session resumes will cause a full cache rewrite).

However, cache being hit doesn't necessarily mean Anthropic won't just subtract usage from you as if it wasn't hit. It's Anthropic we're talking about. They can do whatever they want with your usage and then blame you for it.