And he is an authority on US nuclear primacy as well?
There are several techniques to detect nuclear warheads. It would be foolish to assume that US is using only a single type of sensor to detect such weapons. Such developments are largely classified. B/W you should read cited content properly:
A series of satellites planned to begin deployment in 1989 (temporarily delayed by the Shuttle explosion) are now aloft (2nd phase of the MILSTAR Program) They carry special sensor devices (Developed by SAIC) that can detect high-velocity spin-off particles from enriched uranium (necessary for nuclear devices). Due to the small size and velocity of these particles, no amount of shielding can block them: not lead, not earth (sub-terrainian). Radiation hazards from these particles are minimal due to limited quantity. Our satellites are fool-proof in detecting and pin-pointing the locations of enriched uranium throughout the world.
Source:
US Satellite Detection Of Portable Nuclear Weapons
Now, how do you think that US sources are finding out about the number of Pakistani warheads? Pakistani authorities do not disclose the size of its nuclear stockpile to the public. Fact is that US is continuously tracking nuclear-related developments in the entire world.
And as I pointed out, nuclear warheads are likely to be kept in hardened facilities that also happen to be heavily guarded. Therefore, it is not difficult to determine their location with a decent surveillance system.