In article , dg@mthost1.mt.att.com1019MT
says...

>the 105mm is not intended to be a copy lens. the
>60mm is flat field, the 105 is not. not sure about
>the 200, but i doubt whether flat field is important
>for its application either.

Try shooting a very flat evenly detailed subject wide-open with
the 105 Nikkor macro at about a 45 degree angle, running the line
of sharp focus diagonally from one corner to the oposite corner.
If you draw a line through the sharp focus on the image, and that
line is straight, the lens is flat-field at that focus distance.
Since the design of the 105mm Nikkor macro lenses allows for
compensating the field flatness throughout the lens focusing range,
I would be surprised if you came up with anything but straight lines.
Hope this helps.