Their clients should talk the talk.
If there are a ton of 5-star Google reviews for a trainer, there’s a reason. If there’s a handful that seems like their best friend’s mom wrote them, there’s a reason.
Look for pictures and testimonials from people who’ve worked with them. If you’re working on dropping 50lbs, and every picture is someone with shredded abs transforming into someone with a tan and slightly more shredded abs, do you think this trainer is right for you? Get proof. Word of mouth counts too.