CRestUtil::isAdmin

static function isAdmin(?int $userId = null): bool
{
	global $USER;

	if ($userId > 0)
	{
		if (ModuleManager::isModuleInstalled('bitrix24'))
		{
			return $USER->CanDoOperation('bitrix24_config', $userId);
		}

		return in_array(1, CUser::GetUserGroup($userId));
	}

	if (ModuleManager::isModuleInstalled('bitrix24'))
	{
		return $USER->CanDoOperation('bitrix24_config');
	}

	return $USER->IsAdmin();
}