This topic has been discussed numerous times before, perhaps you want to check the Database for the answers.
One possibility is to type DSPSFWRSC and then F11 where you can see all the installed versions of the software on your system.
The fastest way I know, is to use the display data area command on QSS1MRI.
I use this data area in programs when I need to know what the version is for some programing issues.
I use the DSPPTF command. This will display the current release as well as the latest cume ptf installed.
The quickest way to determine the version of the OS is to use the DSPPTF command. Of course you need authroity to use the command. We are at V5R4M5 and that command displays it correctly.
That Data Area shows V5R4M000