I think the first thing you have to do is just talk shop with him/her for a while. If you like what you hear, maybe jam for a while. Depending on the price, you should just take some lessons from them, probably 3 to 5 lessons to get a good feel for what they are like.
To find a great bass teacher is very rewarding. I was blessed to have a very talented teacher. We started out using a book, but I picked up the fingering coordination (sp) quickly, so mostly we would just jam and he would show me different techniques.
He cut me loose after a little more than a year. He said I had to create my own style and he had taught me what I needed to make my own sound and style. That was 5 years ago, and I miss having lessons with him, but I'm glad he cut me loose. It made me more creative.