Lilith doesn't mention it. She just presses her fingers to it gently, feels the warm thrum, and smiles faintly.

☀︎

There's been other little details too. 
Candles burn strangely around Tara, almost as if they don't burn at all. Because that's another thing, they don't use flash bulbs to light the room, they use candles. Lilith can't recall if they've ever replaced any of the candles.
They mostly keep the dorm dimly lit. They've hung curtains with psychedelic designs. Sometimes there's herbs on the windowsill, once there was salt. 
Lilith also briefly glimpsed a grimoire on Tara's bed. After Lilith returned from her lecture, the book was gone. 
Once, Lilith played Fleetwood Mac, waited to see if Tara would make a comment about Stevie Nicks. She did. 

☀︎

Lilith didn't confront her. 
Instead, she joined Tara one night for a Wicca club meeting. The others were useless, posers really. Lilith caused trouble. Made snide comments and undermined the fake witches.
But not Tara.
Later that night, walking back to their dorm, Lilith said offhandedly, "You're the only real one there."
Tara didn't deny it. She only said, "You already knew that."
Lilith grinned, like she'd been waiting for the acknowledgment. "I like your quiet kind of power. You should let it be noticed more often."

☀︎

As for how Tara figured Lilith out, well, she noticed early on that Lilith knows things she shouldn't. Runes that aren't in the books. A knack for magical theory with no formal training.
Once, during a minor magical surge on campus (some other student accidentally opening a chaos portal), Lilith didn't panic. She just watched it crackle open, almost bored. "Portal's unstable. It'll eat itself in about 90 seconds. Wanna grab coffee?"
Tara's first real chill didn't come from the event. It came from how indifferently Lilith took it; what she knew about it.

☀︎

Lilith happened to know too much about magic. Like a witch but not quite. If only for her perspective on magic. 
Tara was practicing a basic light spell when it backfired. Harmless but embarrassing.
From her bed, Lilith said, like if it was as easy as flipping a light switch. "You're pushing too much energy through it. Like trying to light a candle with a forest fire."
"I didn't ask—"
"I know. But I'm bored."
She joined Tara, adjusted the placement of her hands and guided her through it. The candle lit.
Tara noticed something. "You've done this before." 
Lilith smirked. "I've done worse."
That day Tara thought Lilith a witch. She was wrong. 

☀︎

Tara realized it a few days later during a tense confrontation—not a fight but a raw moment.