In JSP, how can i check, using JSTL, if certain session attribute exists in request?

This is code in servlet:

HttpSession session = request.getSession(true);
session.setAttribute("user", user);

I am forwarding request to JSP, where i want to check if there is session scoped user parameter attached.

<c:if test="${??? - check if user is attached to request}">
/   /message
</c:if>