The Arduino WiFi Shield has an FTDI compatible connector that enables you to get diagnostic information from the onboard ATmega 32U.
You can connect to the 32U with a FTDI cable :
You can also use a USB2Serial board :
To send messages to the board, use a program like Coolterm to connect to the FTDI port. The board communicates at 57600bps. When sending messages, terminate with a carriage return (CR).
Supported commands and their parameters :
If you send a string or character that is not recognized, you'll get a list of supported commands.