Some stage hypnotists have their own websites, and offer online video clips of prior performances. Although you may be able to get the same performances on videotape, watching them online is a quicker, more convenient way of making an assessment.
Some booking sites offer areas where you can read feedback from previous clients for a particular hypnotist. Bad feedback is an obvious clue to move on, but keep in mind that a lack of feedback might just indicate that the performer hasn't completed their leap to the Web yet.