The version I've read is "Tutte le verità sono facili da capire quando sono rivelate, il difficile è scoprirle!" But that sounds like suspiciously modern Italian to me, so I wouldn't be surprised to find out that it's itself a paraphrase.

ETA: Apparently it was quoted in Criminal Minds, season 6, episode 11, and I suspect the Italian dubbing backtranslated the English version of the show rather than looking for the original wording by Galileo. (Which would make my version above a third-level translation.)

ETA2: In the original version of Criminal Minds, it's "All truths are easy to understand once they are discovered; the point is to discover them" according to Wikiquote. (How the hell did point become difficile? And why the two instances of discover were translated with different verbs? That's why I always watch shows and films in the original language!)

ETA3: And Wikiquote attributes that as “As quoted in Angels in the workplace : stories and inspirations for creating a new world of work (1999) by Melissa Giovagnoli”.