I wonder if there is any existing methodology to do it, except from playing the game for many hours.
All you have to do is test game balance at each level of progression through the game - whatever "level of progression " means for that game.
A formula based design for difficulty progression can work well. Ideas like introduce one new type of opponent or terrain feature per level. Or maybe that's imbalanced - too fast. So introduce a new one every other level. Or x amount of total enemy hit points per level. Of course this sort of requires a level based game to begin with, but it can be applied to RPGs as well. Skyrim introduces high level magic items spells, and opponents based on player level (IE experience level), as opposed to arcade/shooter level (IE dungeon level).