CBL 2. Document flag if data changed

So I have a CBL MutableDocument instance. Is there a public call/method/function/flag that I can check to know if the document has been changed since fetching it from the db? (Without another call to the db.)

No, mutable documents are not connected to the database they came from (in fact, if you create a new one it is completely detached and has no concept of a database at all).

However, what you might be interested in is the document change listener, which will fire back each time a document with a given ID changes inside of the database (more accurately, when all of the transactions have closed after a mutation).