AC coil sensing circuit with A1120EUA-T Hall effect sensor
The magnetic field from the relay will be very small. The magnetic circuit of the relay is designed to be efficient. The core concentrates the field and directs it to moving the armature. This is vital in order to minimize the power required to activate the relay. Your sensor would need to be mounted very close to the relay coil. Alternative methods would be to wind a small coil with a gapped core and place the sensor in the gap. Pass the current that you with to detect through the coil rather than detecting it at the relay. Also you could consider using a spare contact on the relay to detect its operation instead of the hall sensor.