If he or she is in great shape, then it’s a given that he or she knows how to train. Just as important, do the people who train with him or her appear to be getting in better shape as a result.
However, just because a trainer or coach is good at a skill doesn’t mean that they understand how to teach the skill. There’s a guy I went to school with that was a total natural swimmer- didn’t go out for the swim team until like 10th grade, and then caught on so fast he ended up getting a swimming scholarship. We both ended up volunteering at our high school program for a while, and when he was first starting to coach, he had absolutely no idea how to teach a rookie how to improve their stroke. Everything came so easy for him in the water, he had never had to stop and think about how to break swimming down into bits and then build it back up into a full stroke. I wouldn’t have hired him for a pay position at that time.
But he wanted to coach and was willing to put in the effort to learn how to teach, and he’s gotten good enough as a coach to get a head coach job in one of the elite NCAA D2 programs.
And any time this topic comes up, I’m reminded of one of the best pole vault coaches in the Midwest for many a year, who not only never pole vaulted in his life, but became a parapelegic at age 18 after a freak diving accident. But for many years, if you were in western Michigan, and you wanted to pole vault in college, you either had Jim as your coach, or regularly went to Jim’s camps.