James's mother works in a personnel office that uses police stress monitors on their phones. This enables them to tell if people they're interviewing by phone are telling the truth or not. She knew that James was lying to her about the history project from the readout on her monitor. Nerdlich was carrying a miniature tape recorder attached to a tiny microphone concealed beneath his wrist watch. He played the conversation back for Rocky who was not pleased.
Sound waves make minute vibrations on the surface of water.
The principal installed a microphone in the air vent pipes leading up from the toilets to the roof. As the boys talked about their scheme their voices vibrated the surface of the toilet water and the sound waves were carried up the pipe to the microphone where the principal was able to get the goods on them. The bus company had been having trouble with vandalism so placed a hidden video camera the size of a magic marker on the bus James happened to be riding. His actions were duly recorded. Since the police were suspicious of Deke they had his house staked out.