And I would say there is no real way to verify what level of uranium enrichment has been achieved. Inspectors can only see what they are allowed to see. And even then they can’t be trusted to report the truth.
While I generally agree with your take “Inspectors can only see what they are allowed to see” and that reliable reporting from them is probably unlikely, there are almost certainly ways to deduce what level of enrichment is present based upon any number of ancillary factors, such as how much waste/byproduct or non-fissionable U-238 is being sloughed off, or the concentration of certain gasses along the centrifuge chain. The Iranians might have easily let slip a clue without intending to do so. Don’t forget that centifugation has been the primary means of enrichment among friendly Euro countries for 35+ years and the process details are probably extremely well researched and documented. Again, that doesn’t mean the inspectors are reporting accurately, but why are they at 84%? Not 82%? 85%?
If you’ll recall, Israeli commandos took soil samples around the Osirak reactor before the IAF bombed the thing to smithereens [in ‘81?] and were able to determine that the reactor had not yet been made operational; because they did not want a breach from a bombing to scatter radionucleides all over. The chemistry of the enrichment process is extremely well known. Just the reading from a single meter that wasn’t duct-taped over in the Iranian facility could furnish a very strong clue.