You can validate your CSS code to ensure that it is up to scratch with the W3C Jigsaw CSS validator.
More importantly you can test the accessibility of your website using the wave validator (sometimes linking to this website fails to work, if that is the case, Google "wave validator" and click the top link). For hosted websites you can just type in your url. For websites in development you can get the Google Chrome extension here (this link may also not work, either search the Chrome extension store or find a link on the wave validator website after getting to it through Google). Then just host your website locally and click on the extension. It will highlight any errors in the accessibility of your CSS and HTML in the “Summary” section. Going to the “Details” section will allow you to go through them one by one highlighting each part that requires fixing. This includes aria-labels and colour contrasts.