Sven's correct. You should be looking at the API explorer (https://developer.adobe.com/workfront/api-explorer/) to see what you can, and can't report on.
In the API explorer, custom forms are located under category, fields are located under parameter, section breaks are parameter groups, and category parameters are the exact location and specific detail of a field on a form.
0) Disclaimer: I don't want to dance around the fact that Workfront has very kindly provided a quick way to report on the Forms and Reports each field is used on. I'll just mention these are available and very much appreciated, but there's no way that I know of, to tap into or use the columns further.
1) there's no such thing as a category parameter report, which is where you would get all this grouping ability (the 1:1 match). Looking at the category parameter section, you'll see there's a 1:1 tie, back to forms, fields and sections.
2) looking at the parameter and parameter group sections in the API explorer, you'll see there's no link taking you back to a category or custom form (look under the reference and collections tabs).
3) looking at the category section in the API explorer, you'll see there's an ability to report on the category parameters as a collection. However since this is a collection you'll have no ability to sort.
So your two options end up being 1 and 3 above.
Option 1: You can use a custom field report and an exists filter to bring in only fields that appear in a specific form. I don't think you would be able to tap into any other information about the form, because there's simply no link to get to that information -- so there will be no section grouping or other form-specific information. You would use this type of reporting if you were more interested in specific field information like the options for checkboxes.
Option 2: in theory you can use a custom form report, and create one or more collections columns that can report on the fields being used and what section they are in. This list will definitely come in, out of order, but you can download it to a spreadsheet and re-format however you need it to be. If it was for one specific form, you would be able to hardcode a few columns, one for each section, and rig up some valueexpressions that basically say "if section ID = ____, list the field name"
Otherwise, yes, making a series of API calls, or downloading all your custom forms into a kickstart and pulling out what you need from that file, would be the other options.
