package flexidentifier {
    import flash.external.ExternalInterface;
    import mx.preloaders.DownloadProgressBar;

    public class FlexIdentifierDownloadProgressBar extends DownloadProgressBar {
        public function FlexIdentifierDownloadProgressBar() {
            super();
            ExternalInterface.addCallback("isFlex", callback);
        }

        protected function callback():String {
            return "true";
        }
    }
}

<s:Application xmlns:s="library://ns.adobe.com/flex/spark" preloader="flexidentifier.FlexIdentifierDownloadProgressBar">
</s:Application>