var loader:Loader = new Loader();
loader.contentLoaderInfo.addEventListener(Event.COMPLETE, loadingCompleteListener, false, 0, true);
loader.load(new URLRequest("My_AS2_File.swf"));


function loadingCompleteListener(e:Event):void {
    var loader:Loader = e.currentTarget;
    var loadedSWF:MovieClip = loader.content as MovieClip;

    trace(loadedSWF.totalFrames);
}