Sébastien Thomas has a recent post to his blog shows how (with just a bit of code) you can monitor your APC cache usage on your local host.
I won't go deep into how OpCode caches work, you will find a lot of docs, just google for APC, Xcache, eAccelerator... What I can say is that APC (as other caches do) will save binary parts of your PHP code into memory and use it when you call for the same PHP function again. This way you save all the PHP file opening, parsing, etc. [...] One thing you won't come accross often on internet is How (the hell) do I know if APC is performing well ?
His answer comes in the form of code based on the APC-PHP-Monitor script. He changed it slightly to allow for use in other places, like with Nagios monitoring. A Nagois template is included to query the cache and report back a hit ratio. An example of the full cache results can be seen here complete with general cache info, hits, misses and file versus user caching stats.