Memcache is an awesome caching server but managing it is a bit more complex than most people think. It used to have quite many bugs in its history plus the algorithms of memory allocation and replacement of items might cause confusion. To manage it efficiently you have to monitor hits and misses ratio:

Then another important information you want to track is evictions rate as climbing evictions might indicate that you cache too much or have not enough memory.

Then you want to monitor if server is under healthy load so you should monitor how much data is transfered:

In addition its makes sense to track how many set and gett commands are issued to our server:

And you can monitor concurrent connections cout just to make sure they are not piling up or that there are no errors in the application making too many connections or hogging them for too long

To understand your PHP application better and to understand if your memcache is performing well you have to monitor it for some time. Without that you can only hope your application is caching properly.

You will also need cacti (tested on 87e) to import them and use (gathering script is a php script that can be placed on any web server and accessed remotely to gather stats). I would also recommend to put them in htaccess protected folder so that file is not accessible from external networks.

UPDATE!

Now after some more work and update you can download a combined set of scripts and templates go to the bundle cacti templates page