To monitor the respect of coding standards, add the Rules Compliance widget to your dashboard:
Since Sonar 3.1, it is possible to check the respect of coding standards on both source code and unit tests code.
The Rules Compliance Index (RCI) gives you an overview regarding the respect of coding standards. To understand how the RCI is computed, see Rules Compliance Index definition.
Use the differential views to monitor added issues.
You can set your coding standards (active checks, severity, etc.) through the Quality Profiles administration page.