Green’s Dictionary of Slang

a police officer or other offical conducting a surveillance.

2000		J.J. Connolly Layer Cake 163: He’s paranoid about ready-eyes is Cody, and the whole area is smashed alive with them.
2006		Guardian 24 Feb. 🌐 ‘It was a ready eye,’ the Dog explained, meaning that that the police had prior information about the raid and had been watching the bank.
2011		J.J. Connolly Viva La Madness 72: He loved [...] the meets in dark places, the ready-eyes, the codes, the phone taps.