Estoy intentando descargar un archivo de Internet y lo he conseguido, pero ahora quiero comprobar si el archivo existe en la memoria interna.

else if (arg0.getId() == R.id.btn_download)
{
    Toast.makeText(this, "download button clicked", Toast.LENGTH_SHORT).show();

    DownloadManager.Request request = new DownloadManager.Request(Uri.parse(names[b]));
    request.setDescription("Downloading..");
    request.setTitle("Futsing Magazine Issue " + (this.mPictureManager.getCurrentIndex() +1) );
    // in order for this if to run, you must use the android 3.2 to compile your app
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
        request.allowScanningByMediaScanner();
        request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
    }
    request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "Futsing Magazine Issue " + (this.mPictureManager.getCurrentIndex()
            +1) +".pdf");

    // get download service and enqueue file
    DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
    manager.enqueue(request);

Los elementos recuperados se descargan en / mnt / sdcard / Download. ¿Cómo verifico si el archivo existe o no usa el código?