`gcc -print-prog-name=cc1plus` -v
This command asks gcc which C++ preprocessor it is using, and then asks that preprocessor where it looks for includes.
You will get a reliable answer for your specific setup.
Likewise, for the C preprocessor:
`gcc
-print-prog-name=cc1`
-v
http://stackoverflow.com/questions/344317/where-does-gcc-look-for-c-and-c-header-files