For sites that have the Data Explorer plugin installed, the easiest approach would be to write a Data Explorer query to get the results. On a busy forum, you might need to refine your criteria to avoid returning too many users.
For sites that don’t have the Data Explorer plugin - especially for sites that don’t have many users - you can find users who aren’t actively posting by going to the site’s users page. On that page, you can select a time period from the time filter that’s at the top-left of the page. You can then find users who aren’t posting by clicking on either the ‘Topics’ or the ‘Replies’ table headers. When the caret icon next to a table header is pointing up, the table will display results in ascending order. Here’s an example from meta: https://meta.discourse.org/u?asc=true&order=post_count.
While the approach won’t tell you which categories users are posting in, it will give information about who is active on the site.