Determining Exception Type

In my application, I am using the global.asax's Application_Error to manage all unhandled exceptions -- essentially, I log the exception and redirect the user to specified error page.  

I have introduced a new entry point into my application that requires two parameters.   If either of the parameters are not available, I throw a custom exception to specifically identify this new error type.   The problem is, I'm not sure how to differentiate in the global.asax between this specific exception [I've defined it as MonthlyBreakdownException] and all other exceptions.      How can I obtain the specific error type [MonthlyBreakdownException] in my code below?

I'm using Visual Studio 2005, C#

Here is my code that I'm using currently:
 
        Exception ex = Server.GetLastError().GetBaseException();
        Logger.LogError(ex);
 
//I'd like to check for my new error here//
 
        if (Session["sessionId"] == null)
        {
            Response.Redirect("SystemTimeout.aspx", true);
        }
        else
        {
            Response.Redirect("LandingPage.aspx?sessionid=" + "none", true);
         }

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