1. Visit the developers' sites. Do they look good? Do they load properly in multiple browser types? Do they function in a way you'd expect your website to function? "Ecommerce, in particular, can require a high degree of technical knowledge and experience to set it up and, in some cases, to maintain it. A good programmer can go a long way towards simplifying the process for the end user."

2. How long have they been in business? "This is the type of the industry where underperformers gets sorted out very quickly. The longer a design firm has been in business, the more likely that they have a track record of satisfied clients."

3. Check references. It seems like an obvious hiring step, however, the best predictor of a developer's success for your business is past success.