Someone was watching the watcher.

While Tyler Clementi’s Rutgers dorm mate was spying on him with a Web cam, Clementi was keeping tabs on his roomie, monitoring his Twitter posts about plans to broadcast Clementi’s date with a man.

Clementi visited Dharun Ravi’s Twitter page 38 times between Sept. 20 and Sept. 22, 2010, the day he jumped off the George Washington Bridge, a detective told jurors at a New Jersey trial yesterday.

That is likely how Clementi knew to pull the plug on Ravi’s computer when the male friend came over for a second dorm-room rendezvous.

Gary Charydczak, a detective in the Middlesex County Prosecutor’s Office, said Clementi saved computer screen shots of his roommate’s tweets, where he ranted about seeing Clementi “making out with a dude” and dared others to tune in for a repeat performance.

Several students have testified that Ravi was organizing a viewing party. One student said the party plans included beer and Bacardi.

A witness said Ravi used his computer to adjust the camera so it was trained directly at Clementi’s bed.

Charydczak said Clementi, 18, saved the screen grabs on a hard drive.

Ravi, 20, faces 10 years in prison on bias-intimidation and invasion-of-privacy charges.