How to know the value of built-in macro in RPM?

Use the --eval switch:

$ rpm --eval '%{_rpmdir}'
/usr/src/redhat/RPMS

The --showrc option...

rpm --showrc

...will dump the complete rpmrc to stdout. This will expose all sorts of interest definitions from the innards of RPM. But Corey's answer is better for finding the value of a particular symbol.