This particular flyby won't actually tell us whether or not Enceladus hosts marine life, microbial or otherwise. But by analyzing the chemistry of the plumes, scientists will be able to determine whether or not the world has hydrothermal activity -- in other words, whether or not water from the ocean infiltrates and interacts with the moon's rocky center. When these interactions happen on Earth, the result is warm water filled with life-supporting minerals.
If Cassini's cosmic dust analyzer (CDA) detects molecular hydrogen in the moon's plumes, hydrothermal activity is a given. The amount of molecular hydrogen, if there is any, will help determine just how active the moon really is.