You got me curious and I started ploughing through the documentation, then the source, then the mailing list archives...all without success.
So maybe I'm wrong, maybe you can't. I seem to vaguely remember there being a way to find out the shared buffer hit ratio though, which is what you need to know (i.e. you increase the shared memory segment until the hit ratio gets up in the high 90% range, say 98%-99% under realistic load conditions).
Let us know what you find out on the mailing list. I'm really curious after striking out while searching myself!
