All those arms-reduction treaties are accompanied by some regime of mutual inspections and checks. Each party visits the other's facilities and counts stuff, as

All those arms-reduction treaties are accompanied by some regime of mutual inspections and checks. Each party visits the other's facilities and counts stuff, assesses production and storage capabilities, paperwork, etc. And compares those results to their otherwise-obtained (read: by spies) data about the other party's weapons counts. Of course it isn't foolproof, and each party can and will maybe try to sneak a few more warheads somewhere. But those inspections at least provide some rough limit on the sneakiness, because if they had more than X% more, we would have noticed or so...