Enqueue script only when side widget is used

by

is_widget_active is what you want e.g.

function sidedish_slide_script() {
 wp_register_script('add-sd-js', get_template_directory_uri() . '/form-slider/jquery-1.8.2.min.js', array('jquery'),'null', true );
 wp_register_script('add-sd-custom', get_template_directory_uri() . '/form-slider/formslider.js', array('jquery'),'null', true );
 wp_register_style('add-sd-css',get_stylesheet_directory_uri() . '/form-slider/styles.css', '','','screen' );

 if ( is_active_widget( false, 'widget_id', $this->id_base, true ) ) {
  wp_enqueue_script ('add-sd-js');
  wp_enqueue_script ('add-sd-custom');
  wp_enqueue_style ('add-sd-css');
 }
}
add_action('wp_enqueue_scripts','sidedish_slide_script');

https://codex.wordpress.org/Function_Reference/is_active_widget