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:
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.