Check if current post is the latest (not last)

Simply query for the latest post and compare IDs:

  function wpse110867_is_latest_post( $post_id, $query_args = array() ){
      static $latest_post_id = false;
      $post_id = empty( $post_id  ) ? get_the_ID() : $post_id ;

      if( !$latest_post_id ){
          $query_args['numberposts'] = 1;
          $query_args['post_status'] = 'publish';
          $last = wp_get_recent_posts( $query_args );
          $latest_post_id = $last['0']['ID'];
      }

      return $latest_post_id == $post_id;
  }

Example Usage

In the main query:

if( wpse110867_is_latest_post() ){
   echo 'This is the latest post';
}

To find the last post of a particular post type:

if( wpse110867_is_latest_post( get_the_ID(), array( 'post_type' => 'my-post-type' ) ){
   echo 'This is the latest post';
}