There are some sites online which contain information on unused content in video games (The Cutting Room Floor being an example). Many of these sites contain GameShark codes to enter to enable or expose this originally hidden content. The Wikipedia page on the Nintendo 64 Game Shark Pro states that there was an in-game code search menu, I have not found any guides on how to use this code search menu to expose memory locations not accessible during normal gameplay.
How would I start searching for unused content in a game? Do I need to use a GameShark device, or if I have a dumped ROM of a game can I run that through a HEX editor?