How to parse webresponse and recognize if is an image or if an xml?

I've a php script that if the user ahs the privilege to download and image the image is sent to webbrowser but if the user has not the privileges and xml with the info is displayed.

phpcode:

if($this->can_download($user->id)){

$id = int_escape($_GET['id']);
$image = Image::by_id($id);

if(!is_null($image)) {
    $page->set_mode("data");

    $page->set_type($image->get_mime_type());
    $file = $image->get_image_filename();

    $page->set_data(file_get_contents($file));

    //WE UPDATE THAT THE USER HAS DOWNLOADED A FILE.
    $this->update_database($user->id, "download");
} else {
    $xml = "<status>\n";
    $xml .= "<info code=\"145\" message=\"Image does not exists.\"/>\n";
    $xml .= "</status>";

    $page->set_type("application/xml");
    $page->set开发者_JAVA技巧_data($xml);
} 
}else {
    $xml = "<status>\n";
    $xml .= "<info code=\"145\" message=\"Yor has reached your daily limit.\"/>\n";
    $xml .= "</status>";

    $page->set_type("application/xml");
    $page->set_data($xml);
}

How can I identify the type of response with vb.net to save the image if it is or send the xml to a parser to use it?

This my vb code:

Dim PerformedUserUrl As String = UserUrl & "?username=" & username & _
                                             "&password=" & password

Dim WebRequest As HttpWebRequest = HttpWebRequest.Create(PerformedUserUrl)

WebRequest.Method = "GET"

Dim webResponse As HttpWebResponse = Nothing
Dim stReader As StreamReader = Nothing

Dim response as string

Try
    webResponse = WebRequest.GetResponse()
    stReader = New StreamReader(webResponse.GetResponseStream())

    response stReader.ReadToEnd()

Catch ex As Exception
    MsgBox(ex.Message)
End Try

Return response

How about checking ContentType property?

If it returns "application/xml", it could be XML. Image, otherwise (based on the server side code that you have given)