qBittorrent official forums

Reading the API documentation, I thought I would be able to make use of torrents_properties() or torrents_info() to figure this out, but it has no property that would point this out.

How does qBittorrent differentiate between private and public torrents?

https://github.com/qbittorrent/qBittorr ... t-trackers
Look at the msg field : "This torrent is private"

The search functionality is somewhat hidden in qBittorrent on the other hand. To get started, select View > Search Engine; this unlocks the search tab that you may switch to. The interface is straightforward and easy to use.

Maybe not. I tried to dig but could not find a similar call.
It's a flag on the Torrent so it should be exposed - maybe no one asked for such functionality before?

Check existing/closed tickets here, please: https://github.com/qbittorrent/qBittorrent/issues
If you can't find anything remotely similar, you may create a new ticket about it.