wp_is_site_url_using_https()

Source

File: wp-includes/https-detection.php

function wp_is_site_url_using_https() {
	// Use direct option access for 'siteurl' and manually run the 'site_url'
	// filter because `site_url()` will adjust the scheme based on what the
	// current request is using.
	/** This filter is documented in wp-includes/link-template.php */
	$site_url = apply_filters( 'site_url', get_option( 'siteurl' ), '', null, null );

	return 'https' === wp_parse_url( $site_url, PHP_URL_SCHEME );
}