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

ANDY say: This is a good question. Put in simple words, we can not distinguish whether a living creature originated on the Earth, or maybe arrived millions of years ago from comets or asteroids. In fact, scientists believe that life on Earth may have arrived from outer space. Some say life started in Mars then arrived to Earth. These theories are named "Panspermia". Here is a link. https://en.wikipedia.org/wiki/Panspermia
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az_lender say: DNA will suggest the degree of relationship to other terrestrial life forms. To suppose that the aliens arrived here (a) without dying and (b) without a visible means of transport seems far-fetched.

About 20 years ago, an article appeared in respected Science magazine suggesting that some Antarctican rocks of Martian origin contained fossil bacteria (McKay, Gibson, et al., 1996). The paper aroused interest, but after a while, the various arguments against it seemed to prevail, and we still don't think we've found any terrestrial evidence of ET life. It's not impossible that such evidence could appear in the future.
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Brilliant Answer say: Great question, and this answer will help you reason throughout the rest of your life.

There was a guy named occam about 700 years ago that came up with what we call Occam's Razor.

All it says is that the simplest answer is oftentimes the correct answer. So, it's much more likely a species to evolve here on Earth than somehow making it here by space ship. The same way for example the thousands of people that would need to keep the secret if we faked the moon landing, or crop circles or anything else.
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Joshua say: Because everything besides some viruses are DNA based. Some bacteria are RNA based.
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CarolOklaNola say: Because their DNA molecules rotate the right and not to the left is how we know, and they also are realted in some way to other life forms on Earth.

"...4. DNA is Right-Handed