The following markup can be used to display the Session ID.

<c:out value="${pageContext.session.id}"/>

The following choose / when / otherwise statement can be used to do something based on whether or not there is a session ID.

<c:choose>
    <c:when test="${not empty pageContext.session.id}">
        Your session ID number is <c:out value="${pageContext.session.id}"/>
    </c:when>
    <c:otherwise>
        Sorry. There is no session ID.
    </c:otherwise>
</c:choose>