Generally speaking you can roughly sort most characters into [b]unpowered or realistic melee[/b] for exactly that, unpowered characters with melee or medieval equipment. [b]Low powered[/b] for characters who stray into the supernatural but not beyond what is reasonable for a character with medieval equipment to fight and potentially defeat. [b]Mid powered[/b] for characters who have magical effects that would allow them to cause a lot of destruction on a city street and defeat numerous medieval opponents with relative ease, [b]high powered[/b] where humans can be crushed like ants and a single attack might devastate a city block, and finally it strays into god territory where you start throwing around primal forces and dropping nukes on cities and countries.

The ways of ascertaining tier are many, it's not really an exact science, more something you judge by eye. Technology and weaponry mess up the balance, as something as simple as a pistol can make an ordinary human capable of attacks that stray into mid powered. It's not too big an issue when two opponents agree on each other's characters and dictate the fight is fair, it's more of an issue for tournament hosts who have to balance a wide range of characters the best they can in freeform.