How to check the scroll bar status is already at top or at end?

The scroll bar is shown when user set "overflow:auto;" and the user can scroll things from top to bottom. The problem is , how can the javascript/jquery check when the scroll bar is either at the bottom or at the top? So that

if (is_scrollbar_top || is_scrollbar_end)
//do some thing..

So are there any funciton/ way to check such status? Thanks

Updated: Not working- using jquery ui dialog

html:

<div class = "dialog" id="dialog" title="Past Issues"></div>

javascript:

$('#pastIssues').click(function (event) {
            var issueString = 'product=Headline&year=2012&month=12';
            $('.issues,#issuesBox').remove();
            var dWidth = $(window).width() * 0.9; 
            var dHeight = $(window).height() * 0.9;

            $( "#dialog" ).dialog({
                    height: dHeight,
                    width: dWidth,
                    modal: true,
                    draggable: false, 
                    resizable: false,
            });

            get_past_issues(issueString,2012,12,event.type);
            return false;
        });