Judoon scanner
Judoon scanners were invented by the Judoon, which they use to identify an individual's species. The scanner can also be used to identify foreign languages as well, so that they could communicate with any species fluently.
The device itself somewhat resembles a red dumbell-sized Sonic Screwdriver. When in use a blue light shines from the end and makes sounds much like a metal detector. The sounds may be how the Judoon interpret the results, as their ears twitch when a scanner is operating. A black marker is on the opposite end, which is used to mark individuals who have been checked.
However, the scanners are not foolproof - Plasmavores could hide their true identity from the scanners by assimilating the blood of a human (or whatever species the Judoon aren't hunting). (DW: Smith and Jones)
Scanners can be increased to a higher setting, forcing a plasmavore to assimilate a second time. Also if a foreign species comes into close contact with an individual (such as through kissing) it can leave mild traces that can be picked up by the scanner - this often results in a long and tedious process used to properly confirm what species the individual is. (DW: Smith and Jones)
