Methods, apparatus, systems and tangible computer readable storage media are disclosed to determine an operational status of a device. Disclosed example apparatus include a first sensor to be positioned a first distance from a wire to measure a first magnetic field associated with the wire, and a second sensor to be positioned a second distance, greater than the first distance, from the wire to measure a second magnetic field, the second distance to reduce detectability of the first magnetic field by the second sensor. Disclosed example apparatus further include a comparator to compare the first magnetic field and the second magnetic field to determine an operational status of the device, and a meter to collect audience measurement data based at least in part on the operational status of the device.