How is the state of a bit position on a hard drive measured?
Like Mark said it's the changes in polarization which is used to encode the data; a magnetic head won't see a static field.

Until some years ago recording was longitudinal, which means the fields were horizontal.  



Growing hard disk capacities required a different way: perpendicular recording. The image shows that you can record bits closer together. Currently hard disks have capacities of over 100Gb/in\$^2\$, and it is expected that this technology can achieve 10 times that.