How can be checked if the hotfix has been applied?

We may use any of the below methods to verify whether Microsoft hotfix has already been installed or not.

Method 1 - Getting Information From Registry (Regedit32)

Using regedit or regedt32, access registry at following key:
   HKEY_LOCAL_MACHINE\software\Microsoft\WindowsNt\CurrentVersion \Hotfix

All hotfixes will be listed here (by their Microsoft KB article)

Method 2 - Getting Information From WINNT Folder

Open the \WINNT directory, ensure to 'Show all files' from the menubar (View-->Options). There will be folders visible $Qxxxxxx$ which are the hidden system uninstall directories for each installed hotfix.

Reference:

http://win-techie.blogspot.com/2011/11/how-can-be-checked-if-hotfix-has-been.html