since 1840 we have been accurately enough to tell how the magnetic feild works, every 300,000 years it flips that hasn't occurred for 800,000 years though, hope this helps.
The strength of the magnetic field was first measured in 1835 by Carl Friedrich Gauss and has been measured numerous times since then, showing a relative decay of 10% in the last 150 years. Currently, satellites use magnetometers to probe the three-dimensional structure of the Earth's magnetic field.
