The ASA generates a syslog for that:
Error Message %ASA-2-321006: System memory usage reached utilization %
Explanation The system memory usage has reached 80 percent or more and remains at this level for five minutes.
•utilization %—The percentage of CPU being used
Recommended Action If this message occurs periodically, you can ignore it. If it repeats frequently, check the output of the show memory command and verify the memory usage. If it is high, contact the Cisco TAC.
you can create a logging list and have the ASA send it by email:
http://www.cisco.com/en/US/docs/security/asa/asa84/configuration/guide/monitor_syslog.html#wp1134510
Patrick