The ultimate difference between Assessments and Declarations is that Assessments reveal facts which the GM presumably already established. In the GM's notes, it would say "Ley Line: requires a Lore Assessment of 4 to reveal" or "Guard: Aspect "Open to Bribery" or "Big Bad is looking to hire an assassin specializing in killing wizards." Those things existed already in the GM's mind or notes.
If the player wants to find something which is plausible (or not) but which the GM *did not* until that moment have in mind or written down, it is a Declaration. The GM is free to give a low difficulty to that check, or even require no roll at all, but it is still a Declaration, not an Assessment.