When we discover a new species on Earth, how do we know they're not aliens from another planet?

When you see DNA depicted as a double helix, you can clearly see that its structure is twisted. That twist makes DNA a "chiral" molecule, meaning it is asymmetric in such a way that a DNA molecule and its mirror image are not superimposable. Examples of chirality are everywhere. Take your hands, for example. For all intents and purposes, your left hand and right hand are mirror images of one another, but no matter how you twist or position either hand, you'll find that it is impossible to orient the two of them in exactly the same way. Chirality is the reason you can't shake a person's right hand with your left, or wear your left shoe on your right foot...."

http://io9.gizmodo.com/5907275/ten-thing...
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say: DNA showing they're related to everything else on earth. DNA at all. There's no reason aliens would have four nucleotides.
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Satan Claws say: Let me give you something to think about: when European sailors first arrived to America they found many new species they didn't know about before because those creatures don't exist over there in Europe. How do you know that Bald Eagles aren't aliens?
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Matt say: All life on Earth started from one single cell organism. If a life form is discovered and it fits into the evolutionary, biological and type of life classifications of life on Earth it is almost certain that it originated here from an evolutionary standpoint. It is thought all life on Earth may have started by microbes landing here during ancient impact events though so we could all technically be 'alien' descended.
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intel say: Ordinarily, there is no riskier step that a scientist (or anyone) can take than to make sweeping generalizations from just one example. At the moment, life on Earth is the only known life in the universe, but there are compelling arguments to suggest we are not alone. Indeed, most astrophysicists accept a high probability of there being life elsewhere in the universe, if not on other planets or on moons within our own solar system. The numbers are, well, astronomical: If the count of planets in our solar system is not unusual, then there are more planets in the universe than the sum of all sounds and words ever uttered by every human who has ever lived. To declare that Earth must be the only planet in the cosmos with life would be inexcusably egocentric of us.