There's one effect/ability that does this, but you've got to be a sprite... or get one!
(All thanks to @DrunkCynic, who put two and two together.)
In all of the core set and the Encounters supplements I can only find one thing that gets you a sort of divination of alignment: the sprite's Heart Sight ability:
The sprite touches a creature and knows the
creature's current emotional state. If the target fails a DC 10
Charisma saving throw, the sprite also knows the creature's
alignment. (MM, p.283)
As a third-level Warlock you can gain the Pact of the Chain boon and take a sprite as your familiar: "when you cast [Find Familiar] you can choose... [a] sprite. (PHB p.107)" And now you're just a touch and a telepathic back-and-forth away from knowing one's alignment.
Spells
There's nothing like the old Know Alignment of my youth. But you can Commune and pester your deity for the information.
Alternatively, you've got Glyph of Warding which can trigger on the alignment. (But process of elimination is slow.) Or you might be able to make a case to a GM that Detect Thoughts plus some skill at interpreting those thoughts might get you to alignment, but we're venturing past RAW, here. (Other spells in the ballpark are even more-problematic to try to apply with any surety.)
Magical Items
There are a few magical items that could be used in a process of elimination, as they will harm one of the wrong alignment. These items are: the Talismans of Pure Good/Ultimate Evil and the Books of Exalted Deeds/Vile Darkness. (The Obsidian Steed has a small probability of also achieving this.)
Many items can only be attuned by one of the proper alignment, but I can't think of a way to force another's attempt to attune. If you could, though, we've got a clear winner: the Eye or Hand of Vecna, either of which will change the user's alignment to NE.
[Over]Kill
A coworker reminded me at lunch of the one I'd missed: kill the target, and follow its soul to its eventual destination. Not very practical, but neither are any of the magical items....
For your reference, an Angry GM article on how vestigal alignment has become in 5e. However, he does have some constructive ideas on how to make alignment more relevant, too. (Be warned: Angry GM's materials are liberally... flavored with rude and vulgar language.)
A filtered search of spells and items that include the unconscious condition on Dndbeyond.com yields only two results.
Sleep effects give you the unconscious condition and include language that involves sleepiness. There are at least two spells besides Sleep that do this:
Eyebite has the Asleep effect that can be imposed on a failed Wisdom save:
Asleep. The target falls unconscious. It wakes up if it takes any damage or if another creature uses its action to shake the sleeper awake.
And symbol has the Sleep effect:
Sleep. Each target must make a Wisdom saving throw and falls unconscious for 10 minutes on a failed save. A creature awakens if it takes damage or if someone uses an action to shake or slap it awake.
Both of these spells require saving throws.
For Monsters that can put a creature to sleep, please see PixelMaster's answer
Best Answer
A low(ish)-level solution:
Use Sending spell to reveal her whereabouts
Using the Sending spell you can send her a message and receive an answer. You can ask, if there was a window and what did she see in the window. You also should ask if she was restrained somehow or chained to a wall.
Pass her a spell scroll of Gaseous Form
From the DMG, page 139:
A familiar (or a trained animal) can pass the scroll through a barred window. Using the scroll the cleric can cast the Gaseous Form spell (PHB page 244):