How can I determine if a file is already uploaded to the server or not?
If you want to see if a file exists, then you can add a listener for an IOErrorEvent.IO_ERROR to the URLLoader alongside your listener for Event.COMPLETE.
var urlRequest:URLRequest = new URLRequest("http://bleh.com/file.php");
var urlLoader:URLLoader = new URLLoader(urlRequest);
urlLoader.addEventListener(Event.COMPLETE, complete);
urlLoader.addEventListener(IOErrorEvent.IO_ERROR, failure);
function complete(e:Event):void
{
    initialize(true);
}
function failure(e:IOErrorEvent):void
{
    initialize(false);
}
function initialize(fileExists:Boolean):void
{
    urlLoader.removeEventListener(Event.COMPLETE, complete);
    urlLoader.removeEventListener(IOErrorEvent.IO_ERROR, failure);
    trace(fileExists);
}