They know it the same way they know everything else that they are ordered to do – they are told by their superiors who were told by their superiors, etc?
Sounds like a pretty odd question unless I’m missing something very obvious.
In any event there are of course stories or isolated troops (solo or more) who didn’t know the war was over and kept on fighting and hiding, sometimes for decades. Japanese cases after WWII are notorious and easily googled.
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