I found this video and the content below it to be an excellent primer on performing idea validation interviews. The author says it’s a big mistake to “pitch” your solution at any point in the process, and don’t we all just want to do that so bad? He lays out 5 questions to ask, and 3 things to never do. Great stuff.
http://customerdevlabs.com/2013/11/05/how-i-interview-customers/