unit testing custom polymer components

I am creating unit tests for polymer project . I am trying to see if there is a way of accessing functions inside polymer html file. My control has a paper input and a function formats the user input. My question is how can I access the function:

<test-fixture id="custom control">   
 <template>
    <custom control id="formatInput"></custom control>   
 </template> 

<script>

  suite('<custom-control>', function () {
    var formatInput
    setup(function () {
      formatInput = fixture('custom-control');
    });

    test('check format', function () {
        formatInput.value = ‘Hello!';
        var formatValue = formatInput.format(formatInput.value  );
      }
    );
  });
</script>