The Galileo spacecraft took measurements of Europa's magnetic field and found that it is an induced magnetic field.
An induced magnetic field is a field created by an induced current.
Faraday's law states that a changing magnetic field creates an electric field.
If a conducting loop was placed in this changing magnetic field, the changing magnetic field would cause a current to flow in the loop.
This induced current would create an induced magnetic field that would appose the change in the magnetic field:
Because Europa's orbiting plane is slightly tilted with respect to Jupiter's magnetic dipole,
as Europa feels changing magnetic field in the radial direction as it moves through this field:
This changing magnetic field can create an induced magnetic field by Europa, and this is exactly what the Galileo spacecraft observed.
The magnetometer on Galileo measured the magnetic field around Europa in different places in Jupiter's magnetic field.
By subtracting Jupiter's magnetic field from the measured field, you can find Europa's magnetic field.
As Europa passed through Jupiter's magnetic field, Europa's magnetic field changed in direction, meaning it must be an induced field.
The observation of an induced field means there must be some conducting material close to Europa's surface, that encompasses the whole planet. The simplest explanation for this would be a subsurface salt-water ocean.
Though this doesn't defiantly prove the existence of an ocean on Europa, it's the strongest evidence we have for its existence.
Because of this strong evidence for a subsurface ocean, Europa is a prime candidate for finding extraterrestrial life.