Mechanism-Editor:

•The Mechanism tab should indicate the Mechanism-Editor is active.

•There is more than one Part - usually.

•There is a Part-Outline around each Part.

•The XY-axes, at the start-Point of each Part, are relatively small.

Part-Editor:

•The Mechanism tab changes to a Part-Outline when you start the Part-Editor.

•You cannot see the Part-Outline.

•The XY-axes are relatively large.

•There is a Dimension for the length of the CAD-Line of a Part that you have added to the model.