No matter what the movies have been telling us for decades, Martians are not likely to be humanoid, sentient beings. They won't have ray guns or space suits. If there is life on Mars, it will be very, very small, and probably not too far up the evolution ladder. Pity.
In order to find such small forms of life, small detectors were necessary. Enter nanotubes, which is a fun word to say. Scientists at the Ames Research Center developed carbon nanotubes, each 1/50,000th the diameter of a human hair, that can conduct heat and electricity. Each nanotube is tipped with single strands of nucleic acid (the "NA" in "DNA") from a microorganism. When it comes into contact with a matching strand, the pair form a double helix and send a faint electrical charge through the nanotubes. This charge is how anyone looking at the biosensor, as the tiny apparatus is called, knows life has been detected.
Sadly, no life has yet been found on Mars, but these biosensors are being put to good use on Earth. Tipping the nanotubes with waterborne pathogens like E. Coli and Cryptosporidium means an analyst can get results from the biosensor in the field within two hours -- no lab work required.