Metaphasic sweep

From Memory Alpha, the free Star Trek reference.

A metaphasic sweep is a type of scan that can be used to detect a cloaked vessel.

When an old Klingon D-7 cruiser attacked the USS Voyager, believing that the Empire was still at war with the Federation, Seven of Nine used a metaphasic sweep to reveal its location so that Voyager could disable its cloak. This worked due to the relatively antiquated technology of the cloaking device. (VOY: "Prophecy")