Honestly, find someone fit, ideally someone who does some kind of competitive athletic, who uses the trainer and recommends them.

Personal trainers can be excellent, but the low end is "dangerously incompetent". The only requirement for starting a service, or getting hired at many gyms, is being buff and knowing how to look competent on some machines. As a result, there are plenty of PTs peddling junk-science nutrition, or giving out plausible-but-harmful workout routines that undervalue stretching or produce dangerous imbalances and overextensions.

Assuming you don't need someone teaching you how to be healthy, though, things get easier. Some trainers focus on education and drop unmotivated clients, but some offer "willpower pill" as a decent chunk of what you're paying for. Knowing that someone expects you at the gym at 7PM, and will pump you up to not bail early, can do a lot of good.

Honestly I think one of the easiest ways is to search https://www.precisionnutrition.com/fitness-pros-directory. You need to pass their nutrition course, which is very good on nutrition. It also costs quite a pretty penny, so if they're willing to spend that on training themselves and have passed it, there is probably a pretty good bet that they're good.

willpower pill is literally what a personal trainer is