Have you noticed a sag in your drywall ceiling?
Is the rest of the ceiling potentially going to sag over time?
Infrared thermal imaging is a great tool to detect drywall screws (see photo below). Recently a local designer called regarding a renovation project he was putting together for a client. He was interested to know how the ceiling joists were layed out (and if they were placed according to local regulations), and what fixings were used behind the wall.
The infrared photo below demonstrates how a thermal imaging camera can yield instant results in most cases… as long as the target is in direct line of sight.