Cameron's trouble with the metal detector in "The Turk".
Metal detectors are an effective technology for screening out Infiltrator terminators with a metallic endoskeleton or body. This includes the entire line of Series 8xx terminators, along with the Mimetic Polyalloy based T-1000 and T-X series.
In "The Turk", Cameron set off a metal detector at her high school. John Connor explained to the security guard that she had a metal plate in her head. This was the first TV or film display of this simple technology to detect terminators (previously, it's been implied that the only way to detect a terminator is by using terminator-sniffing dogs).
Metal detectors were also used to detect terminators in S.M. Stirling's novel, T2: Infiltrator.