Even if you do establish a procedure, how do you guarantee that the reported number of weapons prior to dearmament is accurate (and thus matches the number of dismantled weapons), and that the other side didn't keep a stockpile in an off-the-books location? That brings us back to trust -- that the other side is legitimately engaging with the process and that the totality of weapons is known, rather than merely reducing their numbers (which would be no bad thing, but significantly different to total disarmament, particularly if only one side goes through with it)?
> the other side didn't keep a stockpile in an off-the-books location?
This seems an implausible strategy on the long run, to organize such a conspiracy you would need thousands of people in the know, from political leaders to technicians. The value of this intelligence would be so colossal that it's unfathomable nobody would sell it for profit or release it for conscience reasons. See the Israel story and Mordechai Vanunu.
Then again, if you manage to have a stockpile of nuclear pits (actual warheads require continuous maintenance, Tritium replenishment etc.) that only 15 people know about, in an non-deployable configuration and hide them so deep that the ultra-sensitive nuclear detectors mandated by the treaties can't sense them, is that a major threat? Their existence might simply fade out of institutional memory.
