As Chekhov once said, if in the first act you have Europa or Titan, then in the following one it should turn out to have sentient life.
And as Austen once said, it is a truth universally acknowledged that a single moon in possession of a good ocean must not be in want of sentient life.
My corollary is that you should present said sentient life as a surprise twist ending, because never before has anyone ever speculated about Europa or Titan supporting sentient life, so your readers will astonished.
But seriously: Can't we just take it as read that in sf, there's sentient life on Europa and Titan? Can't we skip the long process in which characters encounter hints of such life, figure stuff out, wander around, and eventually at the end of the story discover proof? I've lost count of the number of stories I've read (mostly published) in which that happens. Apparently writers and editors and readers continue to consider it an interesting plot, but it doesn't work for me.
I love a good first-contact story. But a story in which most of the point of the story is discovering that there's sentient life on some planet or moon doesn't usually do much for me, and if it's set on Europa or Titan, it's a foregone conclusion. It's gotten to the point where when I see the words Europa or Titan in a science fiction story, I'm inclined to start skimming, because I can usually predict most of the rest of the story.
(Yes, yes, one can say the same about any number of broad story outlines; I suppose the interesting part is the details of what kind of sentient life the author comes up, and/or the detailed descriptions of what Europa and Titan are like. But for me, those aspects don't tend to appeal much, so I'm left with a plot that I've seen too many times.)
See also my old entry “Planetary intelligences and surprise twist endings,” which wasn't about exactly the same thing, but related. (Amusingly, I posted that almost ten years ago to the day. I'm not sure whether I've seen fewer planetary-intelligence stories since then, but I've certainly seen no reduction in the number of Europa and Titan stories over time.)