I get this question alot. Here is a post of some good questions to ask. I would add to this list: look at the students. This is who you will be. If the instructor is impressive, but the students appear to have trouble, this issue will probably come to haunt you, too. Conversely, you may find a training hall with an instructor who doesn't appear to be a fighting machine, but the students all appear to be performing well. There's alot to be said for subtlety. In particular, one doesn't like to invite attack. Being unassuming in your skill can go a long way toward keeping you safe, though that's something which is hard to gauge when first walking into a dojo.