if you have jQuery and want to detect if something is partly shown, just use this here

jquery.isPartlyInViewPort.js

function isPartlyInViewPort($entry){
    var windowScrollTop = $(window).scrollTop();
    var windowHeight = $(window).height();
    var windowVisibleBottom = windowScrollTop + windowHeight;
    var entryTop = $entry.offset().top;
    var entryOuterHeight = $entry.outerHeight();
    var entryVisibleBottom = entryTop + entryOuterHeight;

    var isAboveViewPort = entryVisibleBottom < windowScrollTop;
    var isBelowViewPort = windowVisibleBottom < entryTop;

    return !(isAboveViewPort || isBelowViewPort);
}