At my hapkido school my instructor is a 3rd dan in hapkido and also a 4th dan in TKD. He tests all his students up through provisional black belt and then his instructor, Master Geoff Booth - an 8th dan bb - adminsters the bb tests.
I think butterfly has the right idea - rank really doesn't tell how effective an instructor someone is, not to mention how good a practitioner he or she is. The only way to tell is to attend some classes and see how the person instructs and how their students' abilities appear to you. My instructor is "only" a 3rd dan but he's an exceptionally gifted athlete who trains diligently, eats a strict vegetarian diet, participates in triathlons, and most importantly is a really, really, nice guy - all attributes that comprise a gifted instructor and practitioner.