The most expensive part of a warhead by far is the fissile material. I imagine the Russians show the US inspectors their stockpiles of fissile material that wou

The most expensive part of a warhead by far is the fissile material. I imagine the Russians show the US inspectors their stockpiles of fissile material that would need to be melted, cast, then machined before it could be incorporated into a functional warhead. There were over a dozen inspections last year IIRC. Nothing known to science produces gamma rays at the rate per unit of mass that fissile material does, so that is how the US would detect sham fissile material. In fact I suspect that the inspectors measure the entire gamma-ray spectrum and would be able to distinguish Russian fissile material from, e.g., Chinese or French fissile material using that spectrum. I imagine so because after a nuclear explosion there is a US agency that can determine from which country's stockpile the bomb's fissile material came from (by measuring isotope ratios in the fallout). My guess is that the inspection regime is not airtight but if the Russians are cheating then it has cost them 10s of billions of dollars (to produce new fissile material).