Cabinet width.
The DD's are either 24" or 27"
belt drive is 29"

The Whirlpool made, standard or super capacity, filter on top dryers are always 29"
wide. Originally a perfect match with the belt drive washer.

Visually, unless a picture has been modified, it's easy to see the difference when comparing side by side.

Of course if one has a look inside the washer, or can see the back then it's even simpler.

-----------

This laundry pair is going on 40 years old and ....it's still going.  

IMO- still looks almost new.  

I remember buying a 1991 pair like this, in almond, as a gift for my mother.