Crooks who make and deploy ATM skimmers are constantly engaged in a cat-and-mouse game with financial institutions, which deploy a variety of technological measures designed to defeat skimming devices. The latest innovation aimed at tipping the scales in favor of skimmer thieves is a small, battery powered device that provides crooks a digital readout indicating whether an ATM likely includes digital anti-skimming technology.

https://krebsonsecurity.com/2017/12/anti-skimmer-detector-for-skimmer-scammers/