(My local opera company puts on related educational seminars within a week or two of any opera that will be performed. For Werther, there was a seminar on mental health, and then the primer this past weekend, which provided an overview.)
In future: in addition to finding out what the story is, you can just try listening to the composer's music (not necessarily the opera in question, but that, too). If you like it or at least find it interesting, you probably will be okay with the opera. Might not always work--I'm thinking the Verdi Requiem, which I love, but I'd never go to a Verdi opera. Also might not work if you don't like the story but like the music (see Sneak and my conversation about Missy Mazzoli's Breaking the Waves). But as they say, forewarned is forearmed.