This mod will allow you to see the level of Players and Monsters. If a definite level cannot be found for a player a level range is giving to you that is based on the highest armor that player is wearing. If a monster has a defense that is incredibly high it's level will be displayed as 500.
For Player Characters:
-Remember that first the level is determined by the amount of defense a unit has (which is set level to level). If that doesn't work then it determines the highest ranking armor and returns a level range.
For Monsters:
-First it checks to see if there is a grabbable level. If there is it will display it. If not the level is calculated from the units defense. If it is a world boss it will be displayed as level 500.
This mod is fairly accurate and should give you a decent advantage of knowing your foe.