Using a Field to Display the Viewport Scale
You can insert a Field and link it to a Viewport object to display, and automatically update, the Viewport's Zoom Scale Factor.
Insert a Block with Attributes, or just create a piece of Text. Edit the Attribute. Right-click the value and choose insert Field.
In the Field dialog (below), set your Field category to Objects, Set Field names to Object, and click the Select Object Button. Select the Viewport. You'll be returned to the Field dialog; choose Custom scale from the Property column, and choose the desired Format. The Preview area will show the results. Clicking OK will update the Attribute.
In this case we see that my Viewport Zoom Scale Factor is 31/256" = 1'-0" which is close but incorrect. Rescaling the Viewport to 1/8" = 1'-0" and Regenerating the drawing updates the Field.
Now you can see at a glance, whether on the screen or on a plot, whether the Viewport scale is correct.