This question already has an answer here:
We've always played that scrolls are easily identifiable. ("Inside the chest you find 200gp and a scroll of fireball.")
Apparently we've been doing it "wrong". The rules say, "If the spell is on your class’s spell list, you can read the scroll and cast its spell without providing any material components. Otherwise, the scroll is unintelligible."
So how, by the rules, do characters identify a scroll?