Enter Action Replay (GameShark) code "8818EDEF 0002" and then press the AR/GS button while on the titlescreen.
If you have enabled the Debug code above, there's a few more debug options that can be unlocked using AR/GS codes
"810DC515 0001" Showing the master display list currently in use.
"800DC663 0001" Showing some bars at the bottom, CPU and other resources.
"8018EDF2 0002" Changes sound mode to "XXX" on the debug menu.