It's been said you are not really an expert in a field until you can write about that field so that anyone can understand it. I totally agree with this statement! Just think about it a second … if you can not explain a topic to "non-experts" without resorting to a bunch of techno-babble, then the topic is probably still something abstract to you … therefore you are not an expert. Let's apply this principle to test your potential web designers.
Ok. So you've created a "short list" of potential web design firms to design your website. Hopefully you've checked their home page (and portfolio) for conforming to W3C web standards using http://validator.w3.org , tested their web pages in a few different web browsers, and above all did not use the "lowest bidder "philosophy (since this will probably cost you much more later).
Now test these web designers to see if they really are "experts." Pick some feature of what you want on your website, and ask them to send you an email explaining how they intend to do it. Be fair … tell them up front that you want to understand how they will do it.
Some web designers may bulk at the idea right away. They are the "expert" … how dare you ask them to explain their actions! Others may charge you a bit for it (you are taking up their time you know), but it would be money well spent to filter out the "want to be designers."