A firm called Browsercam offers a service which will capture images of how a web page looks like via a wide assortment of browser and operating system combinations. If you have a wide range of browsers coming to your web site, this service is the best way to find out what they are seeing. While they do have fees for this service, it certainly is worth the price when compared to the possible embarrassment of having your visitors find the problem.
Of course, you can create virtual computers with different operating systems and multiple browsers if you want to perform the same task. You'll save some money but it will cost you in time spent.