This device can also be called an EMF meter, as it can measure the relative strength of a field using a scale of 10 LEDs.

Magnetic fields can be measured in many ways. The determining factor is the physical property that is to be measured. It is possible to measure the magnetic field strength at a certain position or the magnetic flux through a defined surface.
Teslameter

The magnetic field strength is measured in Tesla or Gauss, hence the name of the corresponding measurement system: Teslameter.
A measurement head is needed for the measurement. Two different geometries are available: sensors and probes (measurement heads with handle).
Both can:

measure field strengths at a given point in space
measure the magnetic field in small areas or gaps
perform rapid, individual static measurements
measure AC and DC fields
create high-resolution magnetic field maps

The basis for all measurements is the Hall effect. The Hall effect is the production of a potential difference in a current-carrying conductor in the presence of a magnetic field. The potential difference is perpendicular to the magnetic field and, for a set current I, proportional to the magnetic field.
Due to this strong directional dependence, it is of utmost importance to choose the right sensor or probe. There are three different types:

Transversal sensors and probes are ideal for measurements of fields between narrow slits, where north and south pole are opposite each other and create a homogeneous field, and on surfaces which are passed by magnets
Axial sensors and probes for measurements inside cylindrical coils or magnetizers
3-axis sensors and probes. They combine the properties of the other two types while being larger. The teslameter can calculate the entire field and the individual x-, y- and z-components

Fluxmeter

With a combination of fluxmeter and Helmholtz coil, additional parameters like magnetic flux, flux density, magnetic field strength, dipole movements and magnetic potential can be measured.
Fluxmeters measure magnetically induced voltage in coils. Compared to teslameters, fluxmeters are more complicated to operate due to the large number of measurable variables.
Typical applications are:

definition of the total magnetization of a magnet
measurement of the magnetic flux within fixed objects like coil cores
dipole measurements of permanent magnets with help of a Helmholtz coil
construction of proprietary coils and their characterization
measurement of high-frequency fields

A fluxmeter uses a conductor loop or coil to identify the perpendicular magnetic flux. The measurement thus depends on the size of the coil. To measure magnetic flux, flux must be generated by movement of the individual components.

00:00 Magnetic field lines
04:51 Magnetic flux & flux density (field strength)
07:00 Field around current in wire
08:17 Motor effect, F=BIL, Fleming’s left hand rule
15:04 Experiment

Experiment to determine the magnetic flux density for a permanent magnet. This video is carrying out the experiment, video 2 (youtu.be/hIJoLiRGjjo) has the analysis of results.

Search Coil Experiment (measure magnetic field strength)
https://www.youtube.com/watch?v=vb8KwB0ANrg&t=0s

Build a circuit that can measure magnetic field strength and measure how field strength changes with distance.

Measuring the strength of a magnetic field
https://spark.iop.org/measuring-strength-magnetic-field#gref

OVERVIEW OF MAGNET MEASUREMENT METHODS
https://cds.cern.ch/record/382438/files/p127.pdf

There’s a magnetic field and you need to measure its strength. But how? Here are some options. Magnetic Compass Back when I was a kid, we had these things called compasses. It’s just a magnetic needle inside a case that is free to rotate. Since a magnetic field can exert a torque on another magnet, […]

Measuring magnetic fields
Magnetic fields within homes can vary at different locations and also over time.
https://www.arpansa.gov.au/understanding-radiation/radiation-sources/more-radiation-sources/measuring-magnetic-fields

Electric and magnetic fields from electricity

Everything electrical, from a toaster to a high-voltage powerline, produces electric and magnetic fields. In Australia, the electric and magnetic fields associated with the use of electricity are generated at a frequency of 50 hertz (Hz) (50 cycles per second).

Both the electric and magnetic fields are strongest close to an operating electrical source. The strength of the electric field depends on the voltage (typically 240 Volts for households) and is present in any live wire whether an electrical appliance is being used or not. Magnetic fields, on the other hand, are produced by electric currents and are only present when an appliance is operating i.e. there is no magnetic field when an electrical appliance is turned off.
Do electrical sources cause any health effects?

There is no established scientific evidence that exposure to the electric and magnetic fields found around the home, the office or near powerlines causes adverse health effects. However, there are some epidemiological (population) studies that have reported a possible association between prolonged exposure to extremely low frequency (ELF) magnetic fields at levels higher than typical and increased rates of childhood leukaemia. Other research including studies on cells and animals has not confirmed these results. On balance, the evidence related to childhood leukaemia is not strong; however people should be aware of the issue in order to make informed decisions.

For 50 Hz fields Public exposure limits (Occupational exposure limits)
Electric field (V/m) 5000 (10000)
Magnetic field (µT) 100 (500)