Read TraceCallbackEvent to determine whether the SQL connection is monitoring communication between the application and the SQL server to which it connects. For example, if the application uses a TSQLMonitor component, that component assigns a value to TraceCallbackEvent, which it uses to monitor SQL commands. Changing the value of the callback would then interfere with the operation of the TSQLMonitor component. 

TraceCallbackEvent is a read-only property. To assign a callback for monitoring SQL commands, use the SetTraceEvent method. 

SetTraceEvent 

Procedural types 

Creating events