In Monopoly, is the amount of money each player has public knowledge? To put it more specifically, if you ask a player how much money they have, are they obligated to tell you?
All transactions are clearly public. You know how much money someone starts with, and every income and expenditure. So, with perfect memory, you could know how much money each person has. But that is not enough to actually answer the question. Just because a value is derivable doesn't make it public. (Settlers of Catan, for example, has hidden hands of resources despite the fact that the resources held by players is derivable information.)
Additionally, if playing on a pre-2008 version, a player would need to reveal how much money they have when tallying their net worth if they land on Income Tax and go for the 10% of net worth option.
The normal game rules do not mention this one way or another.