If you use log4j configuration, you can change the log configuration directly at runtime in:
JBOSS_HOME/server/default/conf/jboss-log4j.xml.
To enable debug logs:
<param name="Threshold" value="DEBUG"/>
To exclude messages from unnecessary classes (server's internal) modify the threshold of these classes to "FATAL".
If you see only ERROR level logs while starting ear on jboss (4.2.2), you have to remove log4j*.jar from your ear and application.xml.