From http://groups.yahoo.com/group/FateRPG/message/16305
Assessments, Declarations, and Maneuvers are all the same action, the way I look at the system. As individual applications, they’re framed differently. Assessments ask the GM to come up with something that in the story-space is already true, just discovered. Declarations empower the player to come up with something that was already true. Maneuvers empower the *character* to assert something new and now true.
Fred
And from Grant: If the GM is amenable, an Assessment may also allow a player character to ‘discover’ an element that the GM hadn´t even thought of previously. In this manner the Assessment works like a Declaration (see below) with the player stating that his character has identified a weakness, Aspect or other feature. The GM sets a Difficulty for the Skill roll to see if the character was correct in his Assessment, or whether he was mistaken. If the roll fails, the GM may wish to impose a temporary Aspect on the assessing character to reflect this, for example ‘Mistakenly believes the security cameras to have a blind spot’.