You can try the following method

 void Uploader_FileUploaded(object sender, UploaderEventArgs args)
    {
        Uploader uploader = (Uploader)sender;

        //test folder is the path to save the file
        string path = Server.MapPath("~/test" + args.FileName);

        //if the file exist
        if (System.IO.File.Exists(path))
        {
            //you want to do
            return;
        }
            
        
    } 

 

Regards

Ken