GottaFix for WannaCrypt?
PowerShell 2.0, 24 20 16 bytes
hotfix KB4012212

-4 bytes thanks to @whatever by removing  -id.
-4 bytes thanks to @DankoDurbić by changing get-hotfix to hotfix.
KB4012212 is the patch for Windows 7. This can be replaced with any KB-code from the linked page of the patch.
Will return the Source, Description, HotFixID, InstalledBy and InstalledOn information when it's installed as truthy value, and will give an error if it's unable to find it as falsey value.
Here is an example of both a truthy and falsey output (so KB4012212 is installed on my machine, but KB4012215 is not):