Serious reply:
Before selecting a coach do some interviews. Find out how they customize individual training plans, how they determine what kind of runner you are and what you need, how they assess progress along the way, and how they adjust if they are not seeing what they think they should.
I happen to have lucked into a coach at one point who really understood that human runners are not simple machines. Some need to do sprints year-round and track intervals regularly, some need to drop hard intervals altogether in the build-up to a marathon and do more long marathon-paced tempo runs, and most are somewhere in between. Some benefit from overall high miles more than really long runs, while some benefit from putting more emphasis on the long run and making other days shorter.
It's also very helpful, but maybe not 100% necessary, to have a coach who can watch a session from time to time to see how hard certain efforts *really* are. We runners tend to bulldoze through hard workouts and report that they felt pretty good, even if they don't.