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Other Questions and Answers address how to check the progress of Spotlight rebuilding its index, previous to El Capitan: Click the Spotlight menu. The menu/panel that appears reports a progress bar if re-indexing is in progress.
In El Capitan, that menu/panel no longer appears. Now a grey box appears in the middle of the screen where we are expected to enter our search words. The re-indexing progress bar no longer appears.