Currently we use a combination of ram cache stats and some http ram cache information to try to determine if the response was served from ram cache. The ram cache stats don't know about http and the entry in ram cache might not be valid. It is possible to have a ram cache hit from the cache's point of view, but not serve the response from cache at all.
The http cache stats are missing a few stats to determine if the response was served from ram. We would need to add stat for ims responses served from ram proxy.process.http.cache_hit_mem_ims and a stat if the stale response was served from ram proxy.process.http.cache_hit_mem_stale_served.
Ram cache stats for reference
Current http cache stats for reference