How to get all selectors by class name that were dynamically appended in vanilla JavaScript?

You can call your check script in DOMContentLoaded event:

document.addEventListener('DOMContentLoaded', function(event) {
  var targets = document.getElementsByClassName('target');
  console.log(targets);
});

Also you can use DOMSubtreeModified event if you're expecting that something will be added during the runtime.

document.body.addEventListener('DOMSubtreeModified', function(event) {
  var targets = document.getElementsByClassName('target');
  console.log(targets);
});

JSFiddle