First use GoodN64 to rename the rom, either with the command prompt or using a frontend such as GoodWindows or GoodGUI. Roms that are good dumps are renamed to [!]. If they are renamed to stuff like [t2], [b2], [f1] etc, then they are bad dumps. Certain bad dumps can be fixed with a Bad2Good patch. You can download those in the download section. Overdumps ( [o2], [o1] etc.) can be fixed with a hex editor. See the sticky in General N64 Emulation for that info.