If you go to the web2py server admin app e.g. http://127.0.0.1:8000/admin and login you should see the name of the app containing the service and an errors link after the name. Click that and you will see the errors page and be able to see the stack trace.
When running as a Python process you can put print statements in your controller to get an idea of what the parameters look like coming into the request. The output should go out on the terminal you ran python web2py.py from. Be sure to comment out the print statements before embedding in a web server such as apache with WSGI or you will get tickets from the print statement.
