I shall answer this old question with new information! I'd like to keep this question relevant for future readers.
On January 17th 2022, NASA released the following statement:
"Scientists today announced that several of the samples are rich in a type of carbon that on Earth is associated with biological processes."
So yeah, chances are something was alive there at one point or another. Gale crater (the massive hole-in-the-ground that Curiosity calls home) is in a favorable location for life.
As James K has brought up, the fact that Curiosity found carbon doesn't change anything. If there is or ever was anything in that soil would require close observation with a microscope to prove it, which the MSL (Curiosity) rover is not equipped with. The NASA source states that since there is carbon, carbon that is usually associated with biologic processes, the likelihood of life on Mars may be higher than what is stated in the other answer, there just hasn't been enough science power of this particular spacecraft to prove such a thing, and therefore this may be partially based on theory. However, life on Mars period is just a theory. Because this question is more about just on other planets in general, I should address those, but Mars is the most famous example of this. If I had to guess, I'd say there's about a 50% chance that there was something in Gale crater at some point, but I think the chances of life on Europa (A moon of Jupiter) are around 60-70%.
As for my favorite mars rover of all time, Perseverance, no word yet on any carbon, bacteria, or anything living whatsoever. But I believe that Perseverance will find something of this nature sometime during its mission since it is equipped with a microscope. And, let's face it, Perseverance is just so much better than Curiosity (No offence, my robot friend Curiosity, but you're just no longer the new shiny thing).