The inspections are more to check that there aren't more than "a reasonable number" of warheads. With a decent scope to check that excess warheads haven't been sequestered out of sight, and monitoring the scheduled decommissioning of ones to make sure that it's a not a form of fake decommissioning. That and tracking the current and future production lines and critical supply chains (directly and remotely) to cover the possibility of a "hidden arsenal" as much as one can.
You can't really go "so... that active warhead you're planning on keeping: can we just open it up to check that it's not a dud/dummy". Or even "can you just explode <points at random> that one, that one and that one, so we know you're not rocking damp squibs..."
Arms inspections are just geared towards trying to prove an upper-limit of capability, not really a lower-limit. I'm sure there's effort put into this (also trying to work out if diversion of resources is doing funny things to the "what they have, where" official audits), but it's still more of a guessing game. The best info is probably getting verifiable internal (top secret) assessments by common espionage methods, but if your country is wanting to hide a weakness then 'accidentally' getting a fake-strong assessment leaked is probably part of that game, too.
Of course, what really is going on is way beyond my paygrade (one that I'vd never even had in the first place). Yet it'll be something like that, but probably even more layers of obfuscation and sneekiness.