The C++ Users Guide that comes with the compiler explains in detail how the compiler looks for files in #include directives. See especially 12.7 (in the FD6u2 manual) on replacing the C++ standard library. It explains header usage.
You can also use the -dryrun option to CC, and look at the -I options that get passed to ccfe, the C++ compiler front end.
But Forte Developer 6 is obsolete and unsupported. (Some support is still available for udpate 2.)
If you are running on Solaris 9 or 10, please get Studio 12, which is not only supported, but which is free for all uses.
http://developers.sun.com/sunstudio/index.jsp
If you are still on Solaris 8 and can't upgrade yet, you can use Studio 11 instead, which is also free.
By FD6u2 I mean Forte Developer 6 Update 2, also known as Workshop 6 update 2, which I hope is the release that you have. I was referring originally to the C++ Users Guide that comes with that release, section 12.7.
The release also comes with an IDE manual.
If you are talking about the original FD6 (Workshop 6), I don't have access to that release or its manuals. That release is End Of Service Life and no support of any kind is available.