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