1. CSS says: Hello I was (not) loaded

If you have control over how the CSS is loaded you can add events

This extract from my Fabric Iconator checks if the file was not loaded

<link rel="stylesheet" type="text/css" 
      id="FabricCore601"
      onerror="disableMissingFabricCore(this);"
      href="//spoprod-a.akamaihd.net/files/fabric/office-ui-fabric-core/6.0.1/css/fabric.min.css">
</link>

So you can add the onload (for a succesful load) event and track whatever you need to track

2. Hello CSS, are you a DOM element?

LINK and STYLE tags are DOM elements, so like any element you can check if they exist

var myCSS=document.querySelector("link[href*='6.0.1']")

will return undefined if it doesn't exist