When the Master gave Lyra the alethiometer, he told her it would --

From the text:

“It's an alethiometer. It's one of only six that were ever made. Lyra, I urge you again: keep it private. It would be better if Mrs. Coulter didn't know about it. Your uncle—”

“But what does it do?”

“It tells you the truth. As for how to read it, you'll have to learn by yourself. Now go—it's getting lighter—hurry back to your room before anyone sees you.”