In Smallville season 4 episode 11 "Unsafe", Alicia Baker (the teleporting girl from S03E14 "Obsession") is released from Belle Reve and about to start dating Clark again. When she offers him to run off to California and Clart hesitates, Alicia goes to the Torch office and finds the red kryptonite ring.
How did she know how the red kryptonite affects Clark? In "Obsession" she did overhear Clark talking to his parents and learn that green kryptonite makes him sick, but nothing about the red one. More to that, the only people who explicitly knew the effect of the red kryptonite are Clark's parents and Pete Ross, so it's highly unlikely that Alicia could have learned about that in Belle Reve.
As per their dialogue in the episode, Alicia tells Clark that she knows everything about him, but it's never explained.
Clark: What'd you do to me?
Alicia: Okay, don't get mad.
Clark: How'd you know about the red kryptonite?
Alicia: Because I know everything about you, Clark. We were meant to be together, remember? Just like you said.
So, how did she know that she needed to use red kryptonite on Clark?