In the sequels to Arthur C. Clarke's "2001, A Space Odyssey" humans discover, and are told to not interfere with, life forms on Europa, the watery moon of Jupiter. Europa is thought to have some tens of kilometer of water ice over (potentially) 100 kilometer of liquid water, with a rocky core that is kept at least partially molten by the stretching and straining caused by the tidal pull of nearby Jupiter, around which it zips every 3.55 days.
It is thought quite possible that the ocean bottom of Europa, as the ocean bottom of earth, could have hydrothermal vents, and as on earth, these vents could potentially support life. On earth, ecosystems around the vents rely on chemosythesis (the production of energy bearing organic compounds by the oxidation of high energy chemicals, in this case hydrogen sulfide) rather than photosynthesis.
A wide range of methods have been proposed for testing the hypothesis that their are lifeforms in the waters of Europa, from biological testing of the waters to robotic submarines to a search for freeze dried lifeforms orbiting near Europa, thrown off into space by collisions with other bodies.
All of these seem like long shots to me, even assuming there is life there to find. Hydrothermal vent lifeforms on earth do not venture far from the vents. The concentration of hydrogen sulfide drops off rapidly as one moves away from the vents. It is likely that a Europan ecosystem based around hydrothermal vents would be very localized, soaking up every scrap of whatever the chemical was that fueled their physiology. Just as there is very little sunlight reaching the dirt in a mature forest, very little of the chemical energy source would likely escape beyond the immediate surroundings of the vents. The autotrophs would be in tight knots around each vent, and the heterotrophs would tend to follow. Unlike our oceans, in which the entire surface is covered with a sheen of plankton, the top of Europa's waters would likely be dead.
This line of thinking points to two other proposed methods of looking for life. One is to map out beforehand where the hydrothermal vents are, and send a probe directly there. This may be doable, if we can use magnetic and gravitational data to map the plates and faults of Europa's rocky interior's crust, and predict the mostly likely spots.
Perhaps simpler as a first step would be to look for the breakdown products of chemosynthesis in the waters. Just as the earliest sign distant aliens would have had of life on earth was the buildup of excessive oxygen in our atmosphere as a byproduct of photosynthesis, perhaps the easiest way to tell if there are Europans is to look for their waste gases.
This of course, is made more difficult that we have no idea what chemicals they would use as an energy source, or how they would process it. We would basically be looking for any sort of surprising chemical imbalance, with the assumption that we can thermodynamically predict what a balance should look like.
Of course bringing back a Europan in a jar would be much more exciting than saying, " we have a significant deviation from our predicted chemical balance." But checking the chemical balance is probably a lot more doable as a first step.