In Harry Potter and the Philosopher's Stone, while Harry is dreading the upcoming Quidditch match against Hufflepuff because Snape is going to be the referee:
"Harry didn't know whether he was imagining it or not, but he seemed to keep running into Snape wherever he went . . . Could Snape possibly know they'd found out about the Philosopher's Stone? Harry didn't see how he could — yet he sometimes had the horrible feeling that Snape could read minds."
Later in the series we find out that Snape is a skilled Legilimens while he's teaching Harry Occlumency.
— Quora user Nate Kim