The key is to get them talking instead of you. That does help find what their pain points are, so you can address them as the mentalist above noted, but you also find out what they are really looking for so see if you're still interested.

Here's an example of what I mean. Suppose you're a Java/Struts developer historically, but have some serious Ruby on Rails experience recently. You come across a job listing for a RoR developer, with all the right RoR talk in the description with "experience with Java/Struts a plus" at the bottom.

You go into the interview and just nail all the RoR questions. They are happy, you're happy, and they ask you if you have any questions, so you ask them to describe their project. As you play mentalist you discover that they really are using Java 1.4 and Struts 1.38 and want someone to come on and maintain that old code while hoping that some day management will let them rewrite the whole app in RoR. They want you there in case management agrees and to "train" the others in RoR just in case it happens.

They extend you a job offer. You decline and go after a real RoR opportunity with a hot startup. Life is good.