Alltough there are ways to keep your design fitted for all resolutions, there's basically only way you're sure about this. Test it yourself on different resolutions.
One of the solutions can be to design you're website for diferent resolutions (suppose you're basic design is for 1024x768, you could design a 'minor design' fitted for lower resolutions, using Javascript you can determine the users resolution, and if they fail to meet the requirement you can redirect them to the cut down version
If you don't want to be bothered by this... Use a simple message telling the site is designed for the specific resolution.
Anyway, most of the times I design for 1024x768 as this is the most common resolution these days. Based on all sites I've been involved with and I have figures from about 53% use this resolution, and hardly 3-4% use lower resolution. The rest of them are divided in small groups of about 3-4% each in higher resolutions.
If you only take into account for the more graphic related sites, about 45% use the 1024x768 and 25% use 1280x1024. Hardly 1,2% uses a lower resolution (again rest are divided in small groups).
Considering this, if you're designing for 1024x768 it's hardly worth it to give a notice to lower resolution users as they only form a minimal percentage of visitors. If you're going bigger then it might be usefull.
(ps the figures are based on about 25 differentwebsites with an avarage of about 2 000 000 visits per month in total (unfortunately they don't belong to me
) )
