Work on the non-nuclear elements of a bomb design--precisely shaped pieces of conventional explosive, sophisticated electrical devices and circuits, parts made from other metals, deuterium and tritium, etc. could easily be going on at the same time. These components could be assembled and tested by themselves, and measurements from the tests could probably tell their scientists if there probably would have been a fission explosion if the nuclear materials had been in place.
That kind of simulation is not as certain as a full nuclear test, and I'm sure U.S. scientists learned a lot from the various fizzled nuclear explosions that occurred in tests. Still, it must work pretty well, or this country could not have largely eliminated full test explosions. I have no idea if Iran has the technical equipment needed for these simulations to have good predictive power, but I would not want to assume it did not.
Because all this work can be done while the uranium is being enriched, once you had, say, forty pounds of 90+% U-235, it would not take very long to form it into the necessary size and shape and assemble it with the other already-tested components. I think this is what Israel is concerned about--that because the work could proceed fast in the final stages, its estimates of just how much bomb-grade uranium Iran had manufactured would not have to be off by much for it to be too late to prevent it from making a bomb. I suspect that to start to enrich past 20% is to cross the red line Mr. Netanyahu has talked about.
Killing one person is murder, killing 100,000 is foreign policy