This is a method for scanning an open page for class declarations that don't exist in any loaded CSS. This only works for CSS files that are hosted on the same domain as the open page! I tested this code in Chrome. Just load the page you're interested in, open the console, paste in the code, then type `scan()`.

scancss.js

function classExists(className, root = null) {
  var pattern = new RegExp('\\.' + className + '\\b'), found = false
  var sheets = root;
  if (root === null) {
    sheets = document.styleSheets;
  }

  try {
    for (var i=0; i<sheets.length; i++) {
      var ss = sheets[i];
      // decompose only screen stylesheets
      if (!ss.media.length || /\b(all|screen)\b/.test(ss.media.mediaText)) {
        if (ss.cssRules !== null) {
          for (var j=0; j<ss.cssRules.length; j++) {
            var r = ss.cssRules[j];
            // ignore rules other than style rules
            if (r.type === CSSRule.STYLE_RULE) {
              if (r.selectorText.match(pattern)) {
                found = true;
                throw "found";
              }
            } else if (r.type === CSSRule.MEDIA_RULE) {
              if (classExists(className, [r])) {
                found = true;
                throw "found";
              }
            } else {
              console.log("Unhandled CSSRule type: " + r.type);
            }
          }
        }
      } else {
        console.log('Skipping the following sheet because of media type:');
        console.log(ss);
      }
    }
  } catch(e) {}

  return found
}

function scan() {
  var allClasses = [];
  var toScan = document.querySelectorAll('[class]');

  for (var i=0; i<toScan.length; i++) {
    var classes = toScan[i].className.split(/\s+/);
    for (var j=0; j<classes.length; j++) {
      var cls = classes[j];
      if (cls && allClasses.indexOf(cls) === -1) {
        allClasses.push(cls);
      }
    }
  }

  // console.log(allClasses);
  console.log("CSS rules could not be found for the following class names:");
  allClasses.forEach(function (name) {
    if (! classExists(name)) {
      console.log("\t" + name);
    }
  });
  console.log("Done.");
}