Firstly, you need to look at the overall scope of work and establish, "what do I really want from this project?" and most importantly, "why?". Often, people think they just need a professional website designed and their problems will be solved, but it isn't so much about the "what", you need to start with the "why?".
Do you need more sales leads? Or maybe you need to be able to sell directly on your website, or increase sales on your existing site? The deeper you dig, the more you'll begin to uncover and find that a good looking website simply isn't enough.
The website designer you search for needs to be able to deliver on the "why". So the best way to find out if they can, is to tell them why you need a website and ask them what they think you need to achieve this.
If you find them simply asking for a list of requests, run the other way! A professional web designer will be able to offer advice and provide a solution to your desired result, not just take direction and give you a price.
By asking the right questions you can weed out the players from the professionals.