Scientists have obtained most of this information from natural geological activity.
S-waves and p-waves are seismic waves- the waves of energy that travel through the Earth as a result of an earthquake .Both P- and S-waves travel easily through solids, but S-waves cannot pass through liquids, and they cannot pass through the outer core. Scientists know that Earth’s outer core is liquid because s-waves are not able to pass through it. When an earthquake occurs there is a “shadow zone” on the opposite side of the earth where no s-waves arrive.
So, one of the reasons why scientists believe the outer core is liquid is the s-wave shadow zone.