It would easy to tell: (unless they were occupied by only a handful of people) they would be useless as fallout shelters without a lot of ventilation, and you c

It would easy to tell: (unless they were occupied by only a handful of people) they would be useless as fallout shelters without a lot of ventilation, and you can't rely on the wind: you'd need big fans and ducts, and the fans would need to keep running after most of the electricity-generating capacity available in Tokyo is destroyed, i.e., this ventilation system would need its own hardened electricity supply.