If at all possible, buy your monitor from a retail store, rather than mail order over the Web. It might cost you an extra $25, but consider this: many mail order houses will charge you a restocking fee or won't pay for shipping if you have to return a monitor. Shipping a 90-pound monitor will cost you far more than the $25 you "saved."
If you purchase your monitor at a store make sure that there won't be any additional fees or charges if you have to exchange the monitor. Also, make sure that you don't have to try and have the monitor repaired before they'll exchange it. You could be waiting for weeks, only to get the same monitor back (with the same problems), and have to go through the whole process again.
Make sure you can exchange the monitor for any reason. Problems can, and do, appear, even if the monitor is 'working properly.' In other words, there are problems that a retailer might not consider problems, as long as the monitor shows any sort of picture. For example, some monitors have focus problems (particularly in corners). You might be able to adjust the monitor to alleviate the focus problem in three corners, but never be able to fix all four corners. Will your dealer accept this as a good enough reason to exchange the monitor for a new one?
If you are shopping in a store, set the screen to the same resolution that you normally use. Reset the monitor to the factory settings. Measure the viewable area - if the borders are extra wide, it may be hiding other problems. Look for any obvious distortion problems - a maximized window should have nice, straight edges on the sides, top and bottom, and particularly in the corners. Adjust the brightness and contrast as described above. Then bring up a white or solid color screen, if possible (you can set the background image to none in the display properties window or even bring up a blank text screen). Look for any variations in brightness across the entire screen. Monitor brightness can vary as much as 30% from center to corners, but this can cause eyestrain. If you see circular shadows or uneven brightness, move on to another model. Ideally, your monitor should have even brightness over the entire screen. You can then do the business card test to give you an idea about convergence. If you detect convergence problems, don't bother to try and adjust the monitor in the store, just move on to another. Finally, check the specifications. Make sure the monitor falls into the proper Class range for your preferred resolution.