If it's an early Green Day song I can garentee that there'll be a high amount of gain (which I'm sure you already know). The most common way of getting really high squealling from your guitar is to use your pick against the strings and slide that down.
Instead of holding the pick so the wide faces are looking to the floor and celing, turn is so the thin side is instead. Next, place it the gap between two string, usually the E and A before finally sliding it down across the fretboard.
This is a great way of squealling, although I'm not sure if it's actaully what you're after. Try, if not, them I'm stumped...anyone any other ideas?