There is an OS/400 command to display the current user ID - DSPWSUSR. This command resides in library QSYS and can be run from any command line. The default output is to the display, but *PRINT is allowed. The following is an excerpt from the help screen for the command:
"The Display Work Station User (DSPWSUSR) command allows you to display information about your current session. Information shown includes:
* Display station * Number of interactive jobs in session * Interactive job currently active"