Judoon scanner
From Tardis Wiki, the free Doctor Who reference
Judoon scanners were invented by the Judoon, which they use to identify an individual's species
The Plasmavore could hide its 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 also be increased to Setting 2, forcing a plasmavore to assimilate a second time.
