(source: https://jsfiddle.net/Arkni/6602hu7y/)

fiddle.css

.required:after {
  content: "*";
  color: red;
}

fiddle.html

<form id="myform">
  <label for="input1">Input 1</label>
  <input type="text" name="input1" id="input1">

  <br>

  <label for="input2">Input 2</label>
  <input type="text" name="input2" id="input2" required>

  <br>

  <label for="input3">Input 3</label>
  <input type="text" name="input3" id="input3" data-rule-required="#checkbox:checked">

  <br>

  <label for="input4">Input 4</label>
  <input type="text" name="input4" id="input4">

  <br>

  <label for="input5">Input 5</label>
  <input type="text" name="input5" id="input5">

  <br>

  <label for="checkbox">
    <input type="checkbox" id="checkbox"> Make input 3 required
  </label>

  <br><br>

  <button id="mybtn" type="button">Mark required fields</button>
</form>

<pre>
  - Check the checkbox to lake the third input required
  - Fill the first input to make the fourth one required
  - Fill the second input to make the fifth one required
</pre>

fiddle.js

var $form = $("#myform");

$form.validate({
  rules: {
    input1: {
      required: true
    },

    input4: {
      required: {
        depends: function(element) {
          return $("[name='input1']").val() !== "";
        }
      }
    },

    input5: {
      required: function(element) {
        return $("[name='input2']").val() !== "";
      }
    }
  }
});

$("#mybtn").on("click", function() {
  markAsRequired($form.find("input[type='text']"));
});


function shouldMarkAsRequired(element) {
  var rules = $(element).rules();
  var requiredParam = rules.required;

  if (typeof requiredParam === "string") {
    return !!$(requiredParam, element.form).length;
  }

  if (typeof requiredParam === "function") {
    return requiredParam(element);
  }

  return requiredParam === true;
}

function markAsRequired(fields) {
  var field;
  var $label;

  for (var i = 0; i < fields.length; i++) {
    field = fields[i];
    $label = $("label[for=" + fields[i].id + "]");

    if (shouldMarkAsRequired(field)) {
      $label.addClass("required");
    }
  }
}