CCatalogDiscountCoupon::GetList

public function getCuponDetails($code){
    $res = array();
    if (CModule::IncludeModule("sale"))
    {
       $res =  \Bitrix\Sale\DiscountCouponsManager::get(true, array('COUPON'=>$code), false, true);
    }
    return $res;
}

use \Bitrix\Main,
   \Bitrix\Sale;

try
{
   /* @var string Номер купона */
   $code = 'SL-B7S9H-2NRD2O0';

   if ( !Main\Loader::IncludeModule('sale') )
   {
      throw new \Exception("Module 'sale' not installed");
   }

   $arCoupon = Sale\DiscountCouponsManager::getData($code, false);

   if ( !empty($arCoupon) )
   {
      echo "<pre>";
      var_dump($arCoupon);
      echo "</pre>";
   }
   else
   {
      throw new \Exception("Coupon [CODE:{$code}] not found");
   }
}
catch( \Exception $e)
{
   var_dump($e->getMessage() );
}

CCatalogDiscountCoupon::IsExistCoupon