Thank you for the link to the book.
Re: “How can the “SAQ team” determine that one of the 6 leads is out of tune?”
If the antenna is operating correctly, the currents in each download should have a similar magnitude and be substantially in phase. Monitoring the downlead currents would, in principal, tell you if there is a fault. I do not know if the SAQ team monitor downlead current during operation.
The following blog on Alexanderson and the Grimeton antenna system might be of interest: https://www.plextek.com/giants-and-small-antennas/