Lotus Notes has built-in explorer which allows you to see the list of available fields for a given document in the database. Of course, the document must be accessible to the user in one of the views of the database.
To activate the built-in explorer select the document and either go File-> Document properties.

Or left-click the document and select Document properties from the drop-down menu.

Then you will see the screen similar to the one below. Here you could explore the document structure, see what are the actual document fields, the type and content of these fields.

Note 1. There might be more fields you can discover with built-in Lotus explorer than those identified with Exporter for Lotus Notes. The reason is that Exporter list of fields is based on the document template. (This template is called design note.) But during the document life cycle in the database, Lotus Notes could add some more fields to the document.
Note 2. The built-in explorer let you see only part of the information of the note. With Exporter you could however export the whole content of the field, not only the part accessible with built-in explorer.
Note 3. One document may have several fields with the same name. The reason for that is that Lotus Notes split big blocks of information into smaller blocks with the same name but with different sequence number. With Exporter for Lotus Notes you could export only first field out of the sequence.