如果字符串是JSON，则jQuery.parseJSON（）应该返回类型为“对象”的对象，因此您只需使用以下命令检查类型 typeof

var response=jQuery.parseJSON('response from server');
if(typeof response =='object')
{
  // It is JSON
}
else
{
  if(response ===false)
  {
     // the response was a string "false", parseJSON will convert it to boolean false
  }
  else
  {
    // the response was something else
  }
}