Aikikai rank can be verified by simply e-mailing them. I have done so in the past.
Instructor certification is generally left up to the organization. Some do so formally although it does not guarantee someone is good at teaching. Everyone has their own learning style and finding yours and the person who can help you explore it is the biggest challenge. Low ranking instructors can be good instructors and lousy technicians. THe same as high ranking instructors can be good technicians and lousy instructors.
I agree with John. A high ranking teacher can be a poor transmitter of information or a poor "body movement shaper". I recommend looking at the students of that teacher. While a teacher cannot be responsible for how everyone looks, if you look at those who have trained the longest with that instructor, that will tell you something although not everything. I am willing to be judged by the black belts that have trained with me the longest. I like the way they look when they train and I wish I was as good as most of them!