Developers often write tests to make sure their code is working properly, and designers should be thinking along the same lines.
It’s difficult to test aesthetics, but with tools like Spur App you can make sure that your design looks good for people with color blindness and other mild vision impairments. You can also make sure that your design has enough contrast and feels "balanced" on both sides.
Check My Colours is another web-based tool that can help out. Simply type in your live URL, and you’ll get a listing of all the areas where your color contrast can be improved. It’s a bit more technical than simply looking at a black and white image, but it’s good for more detailed analysis, especially on a complex web application that might have lots of dynamic views.