How to tell rank differences?

Let's say I look at 2 judo dojos to go enroll in. The sensei for 1 of the dojos is 30 years old and a 3 degree dan. And let's say that the other dojo I visit is run by a guy who is like Masahiko Kimura, 30 years old and a very high rank 6th or 7th degree dan. How would you tell the rank differences if they didn't tell you? Since if both are legitimately trained, but one is more skilled how would you tell? Let's say both of them perform the Osotogari on me without any trouble, since both can demostrate the techniques effectively how can you tell which is more skilled? Would you have to look at the little things to see who is more skilled?