I would use get_current_screen();
to do this. It returns a WP_Screen object.
You should remember that this function will return NULL if it’s called before admin_init
action.
In your case it should look like this:
$screen = get_current_screen();
if ( $screen && $screen->action == 'add'
&& $screen->base == 'post' && $screen->post_type == 'slideshow' ) {
// do your stuff
}