Are they always unidentified, except for when a Wizard already knows the spell? If the wizard already knows it, is it already identified?
Page 200 of the DMG states:
If the spell is on your class's spell list you can use an action to read the scroll and cast its spell.
So if a wizard were to pick up a spell scroll containing a wizard spell, the wizard would be able to identify it.
Later in the same paragraph it also states:
Otherwise, the scroll is unintelligible.
So if the wizard were to pick up a sorcerer spell,the wizard wouldn't be able to read it.
In conclusion if the spell is on the reader's spell list then the reader would be able to identify it.
Casting spells of levels the spellcaster doesn't have
Page 200 of the DMG again:
If the spell is on your class's spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determine whether you cast it successfully. The DC equals 10 + the spell's level. On a failed check, the spell disappears from the scroll with no other effect.
So for example: If you have a 2nd level wizard who finds a spell scroll containing the spell fireball, the wizard can cast if it can pass an INT check of 13 otherwise it has no effect.
There's nothing in the rules that tells us whether it's immediately obvious what spell a scroll is designed to produce. I see touchstones that pointing us in either direction:
Arguments for "everyone knows what spell's on that scroll"
- Teleological: it's (often) the purpose of scrolls to be used by those other than the author. And they're consumed upon use, so I can't just try it in a safe setting.
- Scrolls are, according to DMG5e p.139, readable by anyone literate. Doesn't that include the 'title' of the scroll? Or at least enough text to get the gist?
- We don't have Read Magic anymore, which from ODD to 3.5 was the required... well, sometimes required... well, at least, useful way to figure out what spell was on the scroll without casting it.
Arguments against "everyone knows what spell's on that scroll"
- Use Identify. That's what it's for. (At least, that's one of the things it'll do.)
- Spells aren't, according to DMG5e p.200, readable by anyone literate. They're unintelligible if the spell contained isn't on your class's spell list.
- Both Scroll of Protection and Spell Scroll descriptions in the DMG tell you what happens when the scroll is read. There's nothing to indicate that the reader has foreknowledge of the effects. So it's up to the DM. "But this interpretation is ludicrous! What kind of a world would we have if...?" I live in a world where you can give me intelligible instructions for a series of actions to take on my computer, and I may have no idea what the end result would be. Why not the same for magic, specifically written to be read/executable by laypersons?
There aren't specific procedures--which did exist in earlier editions--for knowing what the end result of a scroll is.
Most DMs I know from AL will say "you found a scroll of Speak with Animals." There's your answer at that table. But I know plenty of DMs who'll say "you found a scroll" and grin like a Cheshire Cat.