Here we go:
They are ALIENS!
Identifying alien organisms in more equable settings would be a much harder challenge, especially if they use the same basic molecules as familiar life - nucleic acids and proteins. But there is one sure-fire giveaway. The building blocks of proteins, called amino acids, are all lopsided in the same distinctive way. Viewed in a mirror, these "left-handed" amino acids would appear right-handed. Such mirror-image molecules exist, but the life forms we are familiar with don't use them. Most biochemists think it is just an accident that "life as we know it" selected the left-handed version. If this supposition is correct, then there is a 50-50 chance that alien life would have picked the opposite handedness. Such "anti-life" would eat "anti-food": right-handed amino acids and other mirror molecules. This offers a simple way to filter out known life from anything alien. Prepare a culture medium of anti-food and see if anything flourishes. Of course it's a long shot, but it is easy to try, and scientists at the Marshall Space Flight Center are now testing the response of microbes from various extreme environments to a bowl of anti-soup.
http://www.justpacific.com/bits'n'pieces/Life%20before%20life%20on%20earth.html
LOL.... now all we need is a quick and unobtrusive way to detect their presence. Oh wait... all they need to do is open their mouths and spew their venom!