At the health club can ask what the person’s registration number is and check it online at the Commission on Dietetic Registration. If the person is presenting themselves as a registered dietitian and they are not, I would highly recommend you contact the Academy of Nutrition and Dietetics at 312-899-0040.
Further, depending on the state where you live, dietitians may be licensed. What that means is people who provide nutrition or dietetic services must be licensed in order to call themselves a dietitian. I would recommend you contact the state licensure board in your state which can be found at the Commission on Dietetic Registration and they could guide you further