Unless you know the starting point for each individual student, and their medical / mental / physical challenges... I don't think you can. I am working with a guy right now, that when he came in, he had done absolutely nothing in his life that resembled athletics or physical work at all. He has a degree, and works in the educational field at a school. When you get to know him, he has some mental and physical challenges, that are not readily apparent. When he first came, he could stand on the mat casually... I could mark an X on the floor with tape, and ask him to put his left foot on the X. It would take him 6 or 7 tries to get his left foot on the X. He comes regularly to class and we keep working with him. He is slowly learning, and slowly progressing. It would be very easy to come in and watch the class and look at the goofy dork over there and assume he needs to take it more seriously, or do his homework outside of class. They would miss the amount of effort and homework he has done, to get to where he is. The point is that different people have different challenges to overcome to gain physical fitness, to gain flexibility, to gain balance or strength or coordination or timing, or any one of the traits. I have found that it is very easy to snap judge someone... yet very hard to correctly judge them... especially without knowing them. A guy like him, I don't think will ever make our katas look smooth or effortless... or maybe not even combat ready. But, if he gets to the point where he can perform all the katas successfully, I know it will represent quite a bit more work than it would for the "average" person.
I have seen many black belts, from many styles, get into the octagon at get demolished, regardless of the standards held by their ranking boards. I have likewise seen successful MMA fighters get jumped in parking lots or convenience stores and get hospitalized by completely untrained thugs. But then some of those guys that got owned in the octagon where decorated special forces guys with combat experience...
I see black belt or any rank actually, as a personal thing. They are not meant to compare one person to another. Aside from telling you how much of a system they may have seen... they don't tell you much, until you get to know that person and their personal history.
Actually...
I am fairly certain you would be wrong.
I have on a number of occasions spotted neurological abnormalities in an individual, prior to being directly informed. Ranging from a mild barely noticeable palsy, to an extremely high functioning autism spectrum individual.
I used to teach a version of Gracie's bullyproof jujitsu to a couple special needs halfway houses.
Not that I am saying I am fool proof, but it's been about a decade since I was surprised by physical learning disability/disorder in a student.
I think 95 percent of inspectors could achieve the same level of student awareness. It really isn't that hard.
The basis for my statement of telling who is doing their homework, requires some baseline observation.
I never start with the supposition that all the students are the same in skill acquisition rate. That though is only true in the most generic of senses. Skill acquisition falls on a bell curve.
Some only learn with herculean efforts, some are insanely gifted at learning, and then a large body of individuals in between.
The only fair way to ascertain how much progress or pushing through a student is doing... is to actually know that student well enough to know how fast he learns, and see his growth metrics.
The more an instructor does this the faster the ability becomes. Eventually you can do it without a lot of conscious thought.
Within three weeks, I can peg 15 students... and see who needs challenging, and see who is challenging themselves.
I can introduce a new form. Work on it in one class and at the next class I can immediately discern who was practicing it during the several day interval.
Even if it was just 30 minutes each day, I can see the difference.