I think some of the best coaches I have seen have been mediocre at best. And by mediocre I mean when compared to the level he/she is coaching at. If the candidate hasn't at least competed at an international level, do you want him/her coaching the national team? I realize there are exceptions to this.
Coaching requires knowledge beyond the scope of technique. There needs to be proficiency in general management, an experience portion and a sound understanding of the body, its parts and their function.
I was stronger than my first coach by the time I got out of high school. If he were still around today, I would still listen to him. Largely because he was a good coach and I could use the feedback on the quick lifts.
There are some key points in considering what to look for in a strength coach:
1. How often do their athletes get hurt? This is number one. And probably two and three. If, as a coach, your athletes are losing training time due to injuries, then you are doing something wrong. Athletes do not get better when riding the pine, for whatever reason. (or, as the ADA said to me before I actually started in my first AS&C position "If my athletes get hurt because of you, have your shit packed before I run into you."
2. What level have the coaches athletes achieved consistently. Key being "consistently." A star athlete is something one sometimes stumbles across, and they can go far with mediocre coaching. The coach who continually produces high level athletes with low injury rates is the one to listen to.
3. How well does what the coach says (or writes) match up with what we know of good coaching practices? (and this includes issues such as sound biomechanical principles, etc.) If they are flying in the face of everything that we consider to be "the basics" then they need to be crushing it at the national or world level. This is a fairly easy one to observe.
4. Can they get their point across to a broad spectrum of individuals?
5. Are they willing to learn? Coaching is teaching applied physical skills. Which means the coach should be learning as well. If you wind up with someone who "knows everyone else is wrong," well, send them a tube of lube for their ears so they can pull their head out of their ass.
There are others that are important, but these are my top picks.