How can I check the default option values in named.conf?

There isn't any command for BIND 9 that will list all named.conf options and their default settings, but checking the defaults can sometimes be helpful in understanding why BIND is doing something that you didn't expect it to, particularly if you've recently upgraded.

All the options are documented in the Administrator Reference Manual for the version of BIND that you're running. The ARM can also be found in the source code tarball (/doc/arm) and in the appropriate folder in the BIND 9 directory on ISC's FTP site: https://ftp.isc.org/isc/bind9/.

In addition, if you download and build source, there's a tool that will print all the supported options (although it doesn't print the default settings):

bin/tests/cfg_test --named --grammar

But if you've downloaded the source anyway, the defaults for nearly every option are set in /bin/named/config.c -- scroll down to where it says "default configuration" and read from there. For example:

options {\n\
#       blackhole {none;};\n"
#ifndef WIN32
"       coresize default;\n\
        datasize default;\n\
        files default;\n\
        stacksize default;\n"
#endif
"       deallocate-on-exit true;\n\
#       directory <none>\n\
        dump-file \"named_dump.db\";\n\
        fake-iquery no;\n\
        has-old-clients false;\n\
        heartbeat-interval 60;\n\
        host-statistics no;\n\
        interface-interval 60;\n\
        listen-on {any;};\n\
        listen-on-v6 {none;};\n\
...

/bin/named/config.c also includes defaults for 'view' and 'zone'.