Paraphrased from: http://smallbiztrends.com/2012/03/choosing-a-web-design-company.html
1. Do you like their site?
First impressions matter. You wouldn’t go to a dentist you had terrible teeth, right? No, you wouldn’t. You are looking for someone to help you with your first impression, so you need to be impressed with theirs.
2. Do you like their work?
Direct industry experience doesn’t matter — or, at least, shouldn’t matter as much. Even if they haven’t done a slew of sites in your direct industry, don’t discredit it. You know what you like, so what matters is seeing things you like in their portfolio. The work should stand on its own.
3. Does their sales person know their stuff?
Do they need to be the actual developer? No, but they should be able to understand your problems and be able to articulate how their service can address those problems. It’s surprisingly easy to spot people who are just talk. If you’re not overly techy, try looping in your IT department so they can help you make some sense of it.
4. Do you believe their story?
It’s not that people actually try to deceive someone, but if you are having a conversation about your needs and their ability to deliver on those needs, you should just ask yourself simply, “Do I think that they are shooting me straight?” If it’s timeline, budget, technologies, expertise, or whatever, the story needs to add up. If you trust your gut, you will make the right decision.
5. Who does the work?
Sales people are great, but the designers and programmers are the ones actually building your website. Ask where they’re located and if they’re full-time or part-time. What you don’t want is to get into a relationship with a company whose employees are all contractors or working oversees because if there’s a problem with your site, you want it fixed now, not the next time they punch in.
6. Who owns the code?
This one is big. If you don’t own the work at the end of the process you should run, don’t walk, from that firm. You will have more problems in the long term with a company who owns your stuff. You want to know that they are going to be there for you after the launch because you want them to not because you have no other choice in the matter.
In this industry, the adage that you get what you pay for is by far the truest of all truths. There is no checklist or silver bullet when it comes to this kind of decision, but the above will help steer you in the right direction.