В командной строке наберите memcached-tool 127.0.0.1:11211 stats

Обратите внимание на значения bytes и limit_maxbytes, get_hits и get_misses.

Легенда:
pid Process id of this server process
uptime Number of seconds this server has been running
time Current UNIX time according to the server
version Version string of this server
rusage_user Accumulated user time for this process
rusage_system Accumulated system time for this process
curr_items Current number of items stored by the server
total_items Total number of items stored by this server ever since it started
bytes Current number of bytes used by this server to store items
curr_connections Number of open connections
total_connections Total number of connections opened since the server started running
connection_structures Number of connection structures allocated by the server
cmd_get Cumulative number of retrieval requests
cmd_set Cumulative number of storage requests
get_hits Number of keys that have been requested and found present
get_misses Number of items that have been requested and not found
bytes_read Total number of bytes read by this server from network
bytes_written Total number of bytes sent by this server to network
limit_maxbytes Number of bytes this server is allowed to use for storage.

Источник: http://www.php.net/manual/en/memcache.ge...

Спасибо за подсказку, пригодилось!