Life signs detectors are small devices that the Lanteans developed in the Pegasus Galaxy.
Overview
A life signs detector held in a hand.
Hand held devices, they can detect various life signs, but they are incapable of differentiating between species. They are also incapable of detecting the Wraith if they are hibernating. They require the ATA Gene to use, and have a range of one hundred meters. They have also been show to be able to detect energy signatures. (SGA: "Rising", "Home", "The Gift", "The Siege", "Runner", "Vengeance")
There are also city-wide life signs detectors on City-ships, including Atlantis, that can also identify the type of life form (species.) (SGA: "The Siege, Part 2")
Behind the scenes
In Rising, the life signs detectors were clearly able to differentiate between humans (blue) and Wraith (red). In later episodes, they did not have this ability. It may just be that the user can select certain signs on the screen and change their color to red for usability purposes.Or the ability may have been considered too powerful and a deus ex machina so was changed.