For starters, you have to make sure that they are actually good at what they do. If you get someone to design a CSS menu for you and it turns out that they are not very good, you may not be able to know this until your customers start complaining. To avoid such issues, it is often wise to just make sure that the company you are using for web design is actually good at it. Well, you can do this by reading a few reviews and recommendations from other people who have had to do such work in the past.