Typically, builds are first tested in the emulator and then final testing is performed on the target device.
For more information on testing S60 applications, see
Forum Nokia documentation page for testing & debugging & optimization, including, for example:
Nokia Test Criteria for Symbian C++ Applications
An online library
Testing Checklist for Symbian C++ Applications
An online library
The Testing section provides information on how to perform unit tests on the S60 platform.
The S60 C++ SDK provides various additional tools, such as event simulation, with which to test the functionality of your application. For more information, see the SDK documentation.