A Cylon detector is a scientific method of determining whether a subject is human or a Cylon-replicated clone.
Created by Dr. Gaius Baltar, the test is administered by subjecting a person's blood to radioactive material taken from one of the Galactica's nuclear warheads, at the suggestion of Number Six. (Bastille Day) The test results are simple: either "red", indicating Cylon, or "green", indicating human. Yet with a single test cycle lasting approximately eleven hours, Baltar estimates it will take over 60 years (not counting breaks for sleep, food, bathroom, etc.) to test the whole fleet (numbering almost 50,000 people). (Tigh Me Up, Tigh Me Down)
Development work on the Cylon detector had previously been halted when Dr. Baltar was accused by Shelly Godfrey of being a traitor and temporarily thrown in the brig. (Six Degrees of Separation). Baltar's "innocence" was proven, however, by Lieutenant Gaeta, who had previously been ordered by Adama to assist Baltar in the development of the device. (Water) Although considered a military secret, the Cylon detector is nevertheless the subject of constant rumour and innuendo (Act of Contrition).
Curiously, the Cylons themselves (in the form of Number Six) are the ones who initially suggest the construction of the Cylon detector by using the nuclear materials. Moreover, Baltar was only put in charge of the project because of his earlier detection of the fleet's first Cylon infiltrator, Aaron Doral, which was also greatly influenced by Number Six. (Mini-series) So ironically, the Cylon detector is very much a product of Cylon invention.