Experience in working with postpartum clients and verifiable education/certification from an industry-respected organization. That’s a start. Then assuming you are satisfied with these two extremely important credentials, you should look for a “match” in terms of the trainer’s training style, communication style (both listening and talking), and whether you and your wife are comfortable with the answers to whatever questions you may have for the trainer. Remember, you are “interviewing” a fitness professional to work with you on a very important part of your life and well-being — your health. I hope this helps.