a) How do you install and use GNU/Linux on this iMac G4 with OS X 10.3.9 ?
 

a) How do you install and use GNU/Linux on this iMac G4 ?...
http://en.wikipedia.org/wiki/IMac_G4

b) It has OS X 10.3.9 currently
http://en.wikipedia.org/wiki/OS_X_10.3

Well the first step would be to decide which of the Linux distributions you want to run. Do you have experience with any versions of Linux currently?

You are somewhat limited with a PPC machine, but you still have the mainstays like Gentoo, Debian, and Slackware. Last I heard, Ubuntu's PPC was no longer officially supported, but was being kept updated by the community.

If you want a good full feature, still supported OS, I'd try putting Debian (powerpc architecture) on it. Other options might be Slackintosh or Yellow Dog Linux, but I don't have much experience with either.

I'm looking through
http://en.wikipedia.org/wiki/PowerPC
http://en.wikipedia.org/wiki/RISC

Also, how do you go about identifying what versions of GNU/Linux are setup on a system?

You mean what version is already installed? Typically there's a file like /etc/issue, or /etc/version, or /etc/*-release (where the * is SUSE, slackware, debian, redhat, etc). You can also poke through the /proc directory for a similar filename (/proc is the directory of the active state of the system, i.e. what is going on right now). I think /proc/version is the one you're looking for.

take a look at: http://mintppc.org/