[Nekromancer]

dmitry111,
Можно рендомным ключём проверять.

var supportStorage = false,
  supportKey = 'supportKey' + Math.random();

if ('localStorage' in window) {
  try {
    localStorage.setItem(supportKey, 1);
    supportStorage = true;
  } catch (e) {}

  if (supportStorage) {
    localStorage.removeItem(supportKey);
  }
}

alert(supportStorage);