There are treaties between some countries that obligate the signatories to have not more nuclear weapons than specified in the treaty. How does the US know that Russia doesn't have more weapons than specified and vice versa?
What prevents any of the parties from silently producing more weapons than they are allowed to?
If a country puts tanks into a certain territory, you can see it with satellites. Bombers and ships are also big enough to be seen by the satellites.
But if they manufacture more nuclear bombs than allowed and store them in a basement, how will the other party ever notice that they cheated? Nuclear warheads are smaller than tanks or warships, sometimes as small as a backpack.