Extract refreshes are performed by the Backgrounder component of Tableau Server. Each Backgrounder process will have its own log file it writes to so if you had 4 Backgrounder Processes there were would be 4 Backgrounder log files. If you're an Administrator for Tableau Server, there are several Administrative Views to monitor the overall health of Tableau Server. There is a "Background Tasks for Extracts" view and this shows the results of extracts being processed. In the upper left hand corner is an "Extract" filter so you can filter on the one extract you're interested in.