One way to find water worlds beyond our solar system could be to look for minerals — or more specifically, to study minerals mixed with cool lava on exoplanet surfaces. This is because if water comes into contact with fresh lava that's in the process of cooling, it can spur the formation of specific minerals within the lava. So, find those minerals, and you may be getting close to the water that formed them — whether that water lies on an exoplanet's surface or is hidden underground.
Of course, this concept assumes some exoplanets actually have cooled lava that can be examined with our instruments — and therefore have exhibited volcanic activity at some point in their pasts — but the odds are in our favor. Just within our own solar system, we've spotted lava flows on Mercury, the moon, Mars and Jupiter's moon Io. Any rocky world is likely to also have been a volcanic world at some point in its history.
