The Google Labs Browser Size tool overlays a visual guide to fundamentally help you assess whether your audience can see the important bits without the need to scroll (http://browsersize.googlelabs.com/).  The core problem is that while offering some degree of use vertically, if you have a centred column (as this website does) then it shows at your browser width and does not update the visuals.  The other item to bear in mind, is that with the wealth of add-ons and widgets, some people may have a crowded tool and in effect see less vertically than it suggests.  Its certainly a good point to revisit and can be useful to think again on what you want people to get from your site and whether they can see that when they first land at your site.