<body>
 <p id="test1"></p>
 <p id="test2"></p>
 <script>
  var dateTest = new Date();
  var arrayTest = ["i","am","array"];
  var pTest1 = document.getElementById("test1");
  var pTest2 = document.getElementById("test2");
  pTest1.innerHTML = isDate(dateTest);
  pTest2.innerHTML = isArray(arrayTest);

  // Date判別メソッド
  function isDate(date){
   var result = date.constructor.toString().indexOf("Date") > -1;
   return result;
  }

  // Array判別メソッド
  function isArray(array){
   var result = array.constructor.toString().indexOf("Array") > -1;
   return result;
  }
 </script>
</body>

// 結果
true
true