The Curiosity rover can do a lot of things, but nobody is expecting her to find direct evidence of life on Mars. In fact, the hunt for life on the Red Planet has been a pretty stunted one. The last time we really looked was during the Viking missions, which tried to find chemical "footprints" that would exist if there had once been life on Mars, but that could end up on that planet for other reasons, as well. What we got back was a less-than-enthralling "Outlook Hazy. Try Again Later."
Ever since, we've contented ourselves with searching for indirect evidence — assessing the planet for signs that it might once have had the conditions necessary for life to happen. That's important, and it will make direct evidence of life more believable if we ever do find it, but it's not quite the same thing.
But now, DNA sequencing tools have become portable enough (and drilling technology has become powerful enough) that some scientists and Craig Ventner think we could send a probe to Mars which could find buried traces of actual DNA protected in the dirt and sequence that DNA on site.
It's also possible that life hitched a ride between Earth and Mars in their early days. Asteroid impacts have sent about a billion tonnes of rock careering between the two planets, potentially carrying DNA or its building blocks. That could mean that any genetic material on Mars is similar enough to DNA that we have a chance of finding it using standard tests.
Even if we don't, we can set up future sequencers to look for molecules that use alternative sugars or chemical letters in the genetic code. "We're not there yet, but it's not a fundamental limitation," says Chris Carr of the Massachusetts Institute of Technology, who works on the NASA-backed Search for Extraterrestrial Genomes.