If only it were that easy. Curiosity already is a $2.5 billion roving science lab, quite literally. However, it's necessary to have a guiding principle for what you set out to investigate when you decide what equipment to bring to Mars, and in the case of Curiosity that principle has been mineralogy and chemical composition. Overwhelmingly, Curiosity's instruments are designed to be able to determine the mineral makeup of rocks and the chemical makeup of samples. This gives it the possibility of discovering hints of life if it gets lucky, but overall it's not very good at that role.
Partly this is because we still need to know about the mineralogy of Mars, there is a ton we don't know. Improving on that knowledge will mean that we are all that much more able to target the regions of Mars that are more likely to harbor or have harbored life. And then we can send a "life detection" class mission.
However, such missions aren't easy. Consider a few of the challenges. In order to determine the composition of a rock you really don't care about the extreme minority constituents of that rock. If you can figure out what the elemental composition of the rock is within, say, 1% that can be a good day. However, if you want to determine whether or not a sample of dirt contains living or formerly living microbes then you are talking about a teeny, tiny fraction of a fraction of a percent of the material. Which means that you don't just need to break down the material into its major parts, you need to figure out how to focus in your studies to just that small bit of biological material. Which could mean extremely high magnification microscopes, for example, though that has a throughput problem. Or it could mean making use of various experiments to prove the existence of running metabolic activities within living organisms, such as using radiologically tagged nutrients, for example. But these sorts of things are pretty much a crap-shoot, and wouldn't help if the biological samples are no longer living.
Also, a NASA life detection mission requires much higher standards of clean-room assembly and pre-launch sterilization, which add expense and complication to the mission.
This is the sort of question that keeps NASA scientists up at nights. It's a fundamentally tough problem to tackle, especially with just a rover full of instruments and experiments. You can try to design the experiment such that you can show that it's the presence of external samples which show signs of biological activity and not the parts of the machinery itself (e.g. comparing results with and without an external sample present, comparing different samples, etc.) but even that isn't foolproof.
Keep in mind that for unmanned "life detection" missions a spacecraft would not just be constructed in a clean-room but all of its components (as well as the whole vehicle) would be extensively sterilized (at 112 deg. C for about 30 hours, for example). Additionally, the vehicle would be extensively swabbed throughout assembly to search for any amount of biological contamination.
However, more than likely the focus will not be on an unmanned life detection mission but rather on a sample return mission (which would be optimized to try to find samples containing life) or on a manned mission (which would also likely entail sample return). In either case there would be a considerable amount of research resources available to study any samples of Martian life, should it be found, and bring to bear instruments or tests which would unambiguously show it to be of a different origin to Earth life.
Partly this is because we still need to know about the mineralogy of Mars, there is a ton we don't know. Improving on that knowledge will mean that we are all that much more able to target the regions of Mars that are more likely to harbor or have harbored life. And then we can send a "life detection" class mission.
However, such missions aren't easy. Consider a few of the challenges. In order to determine the composition of a rock you really don't care about the extreme minority constituents of that rock. If you can figure out what the elemental composition of the rock is within, say, 1% that can be a good day. However, if you want to determine whether or not a sample of dirt contains living or formerly living microbes then you are talking about a teeny, tiny fraction of a fraction of a percent of the material. Which means that you don't just need to break down the material into its major parts, you need to figure out how to focus in your studies to just that small bit of biological material. Which could mean extremely high magnification microscopes, for example, though that has a throughput problem. Or it could mean making use of various experiments to prove the existence of running metabolic activities within living organisms, such as using radiologically tagged nutrients, for example. But these sorts of things are pretty much a crap-shoot, and wouldn't help if the biological samples are no longer living.
Also, a NASA life detection mission requires much higher standards of clean-room assembly and pre-launch sterilization, which add expense and complication to the mission.