Look in the bottom-right corner of any screen.  The panel # will follow your credit union's CUID, and is usually enclosed in parentheses ().  In this example, the CUID is WU and the panel number is 6142.
 
WU (6142) 2/01/21

You can also click the ? button at the bottom of any CU*BASE screen, then choose About This Application.  The Application Info window displays the panel # you are using as well as the underlying host screen ID (we use that to link to the associated online help topic, among other things), as well as the Tool # used to get to that screen.