You can determine a box's range by its guards, of course, but only the range, not the exact contents (experience, gold, etc.). Because at every Value level, there is magic, experience, troops, and exp.
Sometimes you can estimate which end of the specified spread the guards are closer to—but that's based on the respawn loot, if there are "windows" in the loot ranges.
Added after 7 minutes
For now I'll stick to my point of view, so I suggest an experiment:)
I'll post 10 or more screenshots if you want on any template you choose and 10 on CM or MM6 as the most played. Try to guess—a result of 70% will be considered good, 90% god-tier, and if it's less than 60%, then this theory is baseless.
But on the other hand, exp is placed much more often, so if you say "exp" for all Pandora's boxes, I think you could get about 35-40%.
That is, to prove the validity of the theory, I think 50% accurate guesses of the contents would be enough (obviously, if you guessed the type of loot, it's not hard to determine its specific amount).