No to land potions that must be used at the time of surveying.
Yes to a "Surveyor mechanic" "that can "discover" rarities of a land at any given time during its existence.
The same way as you would level up a building in a guild hall, except your "surveyor" explores your land to find rarer properties that may not have been identified at the initial survey.
This could burn even more DEC over time than a single use only potion; and is an additional mechanic to allow "second hand" plot owners to get value out of land that may be put to market by existing holders.
This is a good move to raise the price of DEC, but it would ultimately devalue the price of rare, epic, and legendary plots, as they will become more common with multiple explorations.
You can tweak it.
Common Lands could be surveyed once.
Rare three times.
Epic four.
Legendary five times.
That way you'd discover the rarity when you "re-survey", or get a conclusive report of "yup, nothing here."