This is as much for me as for anybody else, but I hope visitors to this page find it useful.
Obviously, you can always try SharePoint’s standard verbose logging, but I’ve never found that to be terribly useful for search.
No entries yet…
No entries yet, but since they’re web services you can write on your own, debugging them shouldn’t be intractable.
There’s an interesting post on the MSDN forums that is worth a read. It indicates there is no more FIXML, and that the index is stored in a binary format. Microsoft is apparently creating a tool to inspect the index but who knows how long that might take.
There’s a query tool written by somebody at Microsoft which looks quite handy and is compatible with SharePoint Online as well. Ideal when you can’t login to the server. You can get it from CodePlex. This looks very similar to the MossMan query tool in SharePoint 2013.
Use it when:
You can’t directly login to the server.
You want to see what the index knows about a specific document.
What it won’t tell you:
What specifically the query server sent back to a specific query submitted via the browser.
There’s a suggestion in this deck by Agnes Molnar (SharePoint MVP) that you can use the developer dashboard, but there’s no more detail than that. Looks promising though, at least in a pre-production environment.