All it takes is to load whatever page you're working on in your browser, then click the resize button and see what your page looks like in a smaller window. It's a quick and eay way to check it.
You can't please everyone's resolution & monitor size, but you can make your sites more compatible and look good for most of them anyway.
I have a 17" flat screen that is always set to 1024 x 768. I've checked my pages in other higher and lower resolutions and they all look acceptable to me anyway.