Although the F28/70/100 and the DC9/MD80/90 are similar in layout (rear engines, 5-abreast seating), I find them easily distinguishable by some of the fuselage features, for example the nose:
where the a/c on the left is a F100 and the one on the right is a DC9-50. You'll notice the DC9 has 'eyebrow' windows in the cockpit, while the Fokker 100 has a longer nosewheel leg and bigger doors for the nose gear hold.
As ViveLeYHZ stated, the cabin windows are also different on those models, as the F100's windows are a bit larger and oval in shape, while the DC9's windows are more rectangular and smaller.
Tails are also different - the tailfin of a DC9 has a much smoother contour than that of a F100. Also note the tail cones - the Fokker jets have rear airbrakes, due to which the tail cone is not really a cone, and is much bigger than the tail cone of the DC9. Plus when it comes to larger models of the DC9 (DC9-50, MD80), you'll notice that the tail fins look very small compared to the rest of the fuselage.
But even though these two aircraft are slightly similar in looks, they were both designed in the same era, which means, as FoxBravo put it, not copies of each other but rather of the French-built Sud-Est Caravelle.
[Edited 2005-01-14 11:29:37]