The lpd utility can log many different messages using syslogd. Most of these messages are about files that can't be opened and usually imply that the /etc/printcap file or the protection modes of the files are incorrect. Files may also be inaccessible if people bypass the lpr program.
In addition to messages generated by lpd, any of the filters that lpd spawns may log messages to the syslog file or to the error log file (the file specified in the lf entry in /etc/printcap). If you want to debug problems, run syslogd.