There are many tools available which test various aspects of the JBoss AS.
Which one is the best depends on many thing like money (are looking for open source or not?), operating system and what exactly do you want to monitor.
MonitoringManagementAndProfilingTools may provide you some starting point.
But you might also have a look to the Memory Analyzer (Eclipse Memory Analyzer Open Source Project) or test the AppDynamics (http://www.appdynamics.com/info/jboss-monitoring-tools) or SolarWinds (SolarWinds® Server & Application Monitor)