SpotTest builds a folder of deliberately crafted test files, then runs both NSMetadata and mdfind searches against them to show exactly where Spotlight’s indexing breaks down. Its 1.3 update adds a drop window that reports what the command-line tool file(1) thinks a file actually is, set beside what Spotlight’s own importers decided, useful for the exact moment a document goes missing from search for no visible reason. Free, from the developer behind most of Spotlight’s documented bug reports.