var toString = Object.prototype.toString;
var strLit = 'example';
var strStr = String('example')​;
var strObj = new String('example');

console.log(typeof strLit); // string    
console.log(typeof strStr); // string
console.log(typeof strObj); // object

console.log(strLit instanceof String); // false
console.log(strStr instanceof String); // false
console.log(strObj instanceof String); // true

console.log(toString.call(strLit)); // [object String]
console.log(toString.call(strStr)); // [object String]
console.log(toString.call(strObj)); // [object String]