To show the default include directories used when calling gcc you can manually invoke the preprocessor with -v option:
$ cpp -v
The output will contain something like:
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/include
/usr/local/include
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/include-fixed
/usr/include
To find out the used library search paths of ld (linker) just use:
$ ldconfig -v 2>/dev/null | grep -v ^$'\t'
If invoked as $ ldconfig -v the libraries in the used paths are printed out as well.
I found this quite helpfull as I had problems with unresolved includes.