Hello,
you have nagios plugin:
http://kobesearch.cpan.org/htdocs/Nagios-Plugins-Memcached/Nagios/Plugins/Memcached.pm.html
munin plugin:
http://munin.projects.linpro.no/wiki/plugin-memcache
Some cacti templates:
http://dealnews.com/developers/cacti/memcached.html ,
http://forums.cacti.net/about14605.html
http://www.faemalia.net/mysqlUtils/
both on http://www.netuality.ro/monitoring-memcached-with-cacti/tools/20060802
with some explanations.
Or you can use a simple test to connect to the port memcached is listening.
Ludo