To know the standard

Dear all,
I have a need to find the standard of my system such as POSIX.
How can I know that. Is there any way to find it.
I am using GNU/Linux.

Linux pays some lip service to Posix, but Linux is just Linux. Some folks are working on a standard for Linux that most distros would try to follow but I don't believe it's out yet. Linux comes rather close to Posix compliance, but it is not 100%.

Neo posted a link to the SUSv3 (POSIX) specification somewhere - I cannot find it -
so here is the POSIX specification:

The Open Group -- the Single UNIX Specification Version 3
They ask you to register, but the download is free.
Be sure to remember that Linux varies sometimes from this standard.