Assuming that you're talking about an automated process, you should be able to do this by querying the Last-Modified HTTP header directly from the download site.
Because the official URL is a redirection, you'll need to process that first.
So a HEAD request to http://go.microsoft.com/fwlink/p/?LinkID=74689
returns
HTTP/1.0 301 Moved Permanently =>
Location => http://download.windowsupdate.com/microsoftupdate/v6/wsusscan/wsusscn2.cab
Server => Kestrel
Request-Context => appId=cid-v1:b47e5e27-bf85-45ba-a97c-0377ce0e5779
X-Response-Cache-Status => True
X-Powered-By => ASP.NET
Content-Length => 0
Expires => Sat, 15 Dec 2018 02:19:15 GMT
Cache-Control => max-age=0, no-cache, no-store
Pragma => no-cache
Date => Sat, 15 Dec 2018 02:19:15 GMT
Connection => close
From the Location
field you obtain the download URL, which in this case is http://download.windowsupdate.com/microsoftupdate/v6/wsusscan/wsusscn2.cab
from which you get:
HTTP/1.0 200 OK =>
Pragma => no-cache
Content-Type => application/vnd.ms-cab-compressed
Accept-Ranges => bytes
Server => Microsoft-IIS/10.0
X-Powered-By => ASP.NET
Last-Modified => Tue, 11 Dec 2018 11:39:53 GMT
ETag => "80ca583b4691d41:0"
Content-Length => 482287090
Date => Sat, 15 Dec 2018 02:19:46 GMT
Connection => close
X-CCC => US
X-CID => 2
Cache-Control => max-age=0
When the Last-Modified value changes, a new version of wsusscn2.cab
is available. Note that this file is updated infrequently, usually only once a month on Patch Tuesday when new security updates are released, so depending on your scenario it is probably sufficient to check at most a few times a day. There might be occasional false positives, if this is a problem for you you should also check the date on the digital signature of the downloaded file.
If I've misunderstood and you're not talking about an automated process, you can sign up for Microsoft Technical Security Notifications. These notifications won't explicitly tell you when wsusscn2.cab
has been updated, but they will tell you when new security updates are available, and that's almost the same thing.
The Security Update Guide also provides an API.