Judoon scanners were devices used by the Judoon to gather readings.
The device itself somewhat resembled a red, dumbbell-sized, sonic screwdriver. When used, a blue light shone from the end and made sounds much like a metal detector. A black marker was on the opposite end. This was used to mark individuals who had been checked. (TV: Smith and Jones)
The scanners could be used to identify an individual's species. It could also identify foreign languages so the Judoon could communicate with any species fluently.
Plasmavores could hide their true identity from the scanners by assimilating the blood of a human or other species. If a foreign species came into close contact with an individual, such as through kissing, it could leave traces that could be picked up by the scanner. At least once this resulted in a long and tedious process to properly confirm the species of the individual. (TV: Smith and Jones)