Is the photographer nervous? Does she seem disorganized? Is he unsure of his own packages and prices? Does she have a difficult time understanding your desires? Does he not excite you about your big day and the possibilities when it comes to imaging your excitement? Is the presentation material a copy of a copy she got at Kinko's, or did she take the time and effort to create professional material? Keep these items in mind when interviewing your photographer. If most or all of these questions do not crop up, you probably have a winner.
Confidence comes from experience. Experience comes from making enough mistakes to know when not to make them again. The only way to learn is to do, and after 19 years, I've done enough to know when a mistake is about to happen. At least I hope so.