Hi
normally only RST from the rserver or no response to SYN from the rserver are counted as failure.
However, we had issues with this as identified in CSCtd22008 "ACE- Client RST in End-to-End SSL generates Rserver conn-failures."
An old one is CSCsh14278 "sh serverfarm failure conn incremented for successful connection".
So, if you want to be sure, the only option is to capture a sniffer trace.
If you have a probe attached to a rserver then you can get the details why the probe failed using the show probe detail.
For more troubleshooting please refer to
http://docwiki.cisco.com/wiki/Cisco_Application_Control_Engine_%28ACE%29_Troubleshooting_Guide_--_Troubleshooting_ACE_Health_Monitoring
Regards
Abijith