The general recommendations:

 - use cache plugin for your WP installation
 - reduce PHP memory_limit to ([32..96M][1]) to start seeing PHP memory exceed errors
 - disable useless and new plugins
 - make sure all reporting settings are on
 - reduce max worker processes by hand (3..10)
 - set nonzero MaxRequestsPerChild if you think that PHP/Apache may have leaks (errors in compiled code of PHP interpreter or Apache server)
 - reduce ServerLimit
 - reduce MaxClients
 - use PHP in FPM mode

Specific advices:

 - write a bash script to [measure memory usage][2] or [collect peak][3] and put it in cron. You'll have stats for memory usage of a process over time.
 - or use munin as more advanced solution.


  [1]: http://codex.wordpress.org/Editing_wp-config.php#Increasing_memory_allocated_to_PHP
  [2]: http://stackoverflow.com/questions/3853655/in-linux-how-to-tell-how-much-memory-processes-are-using
  [3]: http://stackoverflow.com/questions/1080461/peak-memory-measurement-of-long-running-process-in-linux