You can use this online tool to check the HTTP Headers and Status code returned by your server for a specific URL.
Bellow there's a list of the more common HTTP Status Codes and their meanings:
200 OK
The request has succeeded.
300 Multiple Choices
The requested resource has different choices and cannot be resolved into one. For example, there may be several index.html pages depending on which language is wanted.
301 Moved Permanently
The requested resource has been assigned a new permanent URI and any future references to this resource should use one of the returned URIs.
302 Found
The requested resource resides temporarily under a different URI. Since the redirection might be altered on occasion, the client SHOULD continue to use the Request-URI for future requests.
304 Not Modified
Indicates the resource has not been modified since last requested.
400 Bad Request
The request could not be understood by the server due to malformed syntax.
401 Unauthorized
The request requires user authentication.
403 Forbidden
The request was a legal request, but the server is refusing to respond to it. Unlike a 401 Unauthorized response, authenticating will make no difference.
404 Not Found
The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent.
500 Internal Server Error
The server encountered an unexpected condition which prevented it from fulfilling the request.
501 Not Implemented
The server either does not recognise the request method, or it lacks the ability to fulfil the request.
503 Service Unavailable
The server is currently unavailable (because it is overloaded or down for maintenance). Generally, this is a temporary state.