Even if we do discover incontrovertible proof that there was once life on Mars, that might not mean we actually found alien life. Ancient microbes from Earth could have colonized the Red Planet millions of years ago.
We usually think of meteor strikes as a one-way process, as giant rocks rain down from space and hit the Earth. But Earth itself can also produce meteors, provided it is hit with sufficient force by an even larger object. These Earth-created meteors can then carry fragments of our planet throughout the solar system, with Mars a frequent final destination for these rocks. Scientists estimate Earth and Mars have exchanged about a billion tons worth of material.
We've already found microbes on meteors that seemingly survived their voyage through space (though it's still hotly debated where they came from originally), and so it's conceivable that microbes hitched a ride to Mars when their patch of Earth became a meteor. If that's the case, then with all due respect to Yuri Gargarin and Laika, Earth's first space travelers were these microbes from eons ago, and they could have been the ancestors of whatever life was able to survive on the Red Planet.
So, is there any way we could actually find out for sure? Let's assume for the sake of argument that there is life on Mars, or at least there once was. In all likelihood, we're talking about the absolute simplest forms of life imaginable, not much more than DNA itself. As long as the DNA was underground, away from the cosmic radiation and ultraviolet rays that constantly scorch the Martian surface, then it could survive quite easily for at least a million years. We're currently designing probes that will try to detect this preserved DNA.
If any of these probes find traces of DNA, then there are two possibilities. The first is that the DNA will look unlike what we find in microbes on Earth - even if it is more similar than not, it might have subtle differences that are fundamental enough to indicate a truly Martian origin. But, if the DNA looks like it came from Earth, then it probably did.
MIT engineer Christopher Carr is currently working on a device as part of the Search for Extraterrestrial Genomes project, which he hopes will be able to identify Martian DNA that's much the same as its Earth counterpart. As he points out, it's worth first trying to find the sorts of life we know how to find before putting all our energy into imagining more alien varieties:
"We think that if there is life on Mars, it could be related to us. If the DNA or RNA is as we know it, then we should be able to detect it with this instrument. We would feel awfully silly if we spent a lot of time looking for something that was very different and didn't spend time looking for something that was very similar. Life could have arisen independently, but that is not the most likely scenario."
The chances for success in this mission aren't great - trying to find something so small on a planet so large with such limited equipment is always going to be involve some pretty low probabilities of success - but if life on Mars is anything like life on Earth, then their project might be able to locate it. And, if it succeeds, then we might have a very good idea why Mars really does have life as we know it.