The quickest way is to open Settings and check the system information. Click Start → Settings → System → About. Under Windows specifications, look at the Edition and Version. If it says Windows 11, then the machine is running Windows 11; if it says Windows 10, then it is not.
Another simple method is to use the Run dialog. Press Windows key + R, type winver, and press Enter. A small window will appear showing the Windows version and build number. It will clearly state Windows 11 if that is the installed operating system.
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hth
Marcin