Only shows shady certifications or education
Exercise science is an actual field -- as in, people spend more than eight years pursuing full-time study to get their doctorates in exercise science, physiology, biomechanics, sports nutrition, and a slew of other sub-specialties. In other words, the background, education, and certifications of your trainer actually matter. Now, I'm not saying trainers need to have a degree in the field to be qualified or effective, but they do need to hold a high-quality certification that proves they’ve prioritized some sort of education in the field.
Looking good in a bikini does not qualify as a certification.
Before you sign a contract, ask for evidence of current certifications. Some of the best include those from the American College of Sports Medicine, the National Academy of Sports Medicine, the American Council on Exercise, and the National Strength and Conditioning Association.
Has the attention span of a 4-year-old
Before you hire a trainer, you need to spend some time performing covert ops. Take an afternoon to watch your potential trainer work with other people. Do your own workout, but position yourself within earshot so you can hear how the trainer acts and interacts with their clients. It may sound a little odd, but as long as you're in a public space and aren't crossing obvious boundaries, it's perfectly reasonable to observe, especially considering the amount of money you may spend on this person.
If a trainer isn’t actively engaged with his or her client -- watching form, providing feedback, or offering encouragement -- it’s probably a bad sign. If you see a trainer abandoning a client, spending more time looking at a cellphone than at a client's form, or laughing with trainers and other guests instead of talking to the client in session, keep looking for a different trainer.