Distinct Fighter levels and Unknowable Class Levels
As per the wording of the ability:
The DM tells you if the creature is your equal, superior, or inferior in regard to two of the following characteristics of your choice:
- Strength score
- Dexterity score
- Constitution score
- Armor Class
- Current hit points
- Total class levels, if any
- Fighter class levels, if any
So, for the two things the fighter can learn, total class levels and total fighter levels are valid options to learn. But this does NOT tell you the total levels in other classes.
E.g. If you asked about the total level of a NPC and the fighter levels you might get back the answer “higher” for total levels and “lower” for fighter levels without being told what those extra class levels are.
Also, remember that the DM only has to tell you if they are superior, equal or inferior, in the chosen criteria, compared to the fighter, not what that actual value would be.