There is no default diagnostics log file. You must set up the file before any messages can be logged. For more information about setting up a diagnostics log file, see Chapter 18, X Server Messages.
To display (and log) font diagnostic messages, set the pref-font-extended-diagnostics parameter to "true" (Setup  ->  Change User Preferences  ->  Fonts  ->  Show Extended Font Diagnostics). 
Table 7-11  pref-font-extended-diagnostics Parameter