In recent versions of Inkscape there is an extension called "Replace font" which can list all the used fonts in the document, find & replace specific fonts, or replace all the fonts in the document or current selection.
To use the tool, navigate to Extensions -> Text -> Replace Font.
On the "List all fonts" tab, if you click "Apply", you'll be presented with a list of all fonts used:
There are also tabs for "Replace font" and for "Find and Replace font".
Note that to use these tools I believe you must copy / paste from the list of all fonts; there is no dialog. Also note that the tools can be applied to the entire drawing or just the current selection by changing the "Work on:" selection at the bottom of each tab.