As has been said, there is no “standard” service for this. But you did not tell what exactly you try to do.
If you want to list the fields that are in a document at runtime then you probably have to create a java service which would recursively traverse the document (IData structure) and collect the paths of the fields.
If however you want to statically analyze a document type then I think that there are some services in WmRoot to do this. I’m not sure what they are but there should be something. Otherwise the developer would not be able to show the document structure. Agood reference would be the package WmDoc which can show the document structure, service signature etc.