The return object looks like:

<sms_deliveryreport>

    <data>

        <messageid></messageid>

        <deliverystatus></deliverystatus> 

    </data>

    <call_result>

        <result>True</result>

        <error /> 

    </call_result> 

</sms_deliveryreport>

Your delivery status will be displayed between the "deliverystatus" tag.