Stuff hooked in wp_head

I’m in a project where we’re using the REST API and PageBuilder by SiteOrigin where i want to attach the CSS from the page builder.
SiteOrigin adds some inline css but also css-files which are hooked by wp_head.
This page builder is built on a grid system “filled” with ordinary widgets. Custom widget CSS is therefore added like this:

abstract class SiteOrigin_Widget extends WP_Widget {
  ...
  function generate_and_enqueue_instance_styles( $instance ) {
    ...
    $hash = $this->get_style_hash( $instance );
    $css_name = $this->id_base.'-'.$style.'-'.$hash;
    ...
    wp_enqueue_style( 
      $css_name,
      set_url_scheme($upload_dir['baseurl'] . '/siteorigin-widgets/' . $css_name .'.css')
    );
  }
}

and i can’t figure out how i should get those files from outside.

Therefore my new approach is; how can I get things that’s hooked by wp_head?