How to determine why ARR decides not to cache a response?

I've enabled failed request trace logging, and checked the list of ARR trace messages but I haven't found anything that indicates on a given response from the origin server whether ARR decides to cache the response, if so, for how long, and (much more important) if not, why not? Is this surfaced anywhere?

There are various existing tools available (YSlow, PageSpeed, many others) to check cacheability of a url/site, but those are much less useful IMHO than ARR being able to log the cause for its decision at runtime (especially if there's some kind of weird intermittent issue).

Thanks!