Machines are good at spotting obvious emotions like smiles, but they're not so hot at detecting the extremely brief microexpressions that reveal when people are covering up their true feelings. They may have a keener eye in the future, though: researchers have developed a computer vision algorithm that magnifies facial expressions, making it possible to catch the tiniest bit of displeasure or surprise. While some humans have a knack for spotting these subtle cues, the algorithm is far more effective in early tests -- you likely wouldn't fool the computer into thinking everything was hunky dory.
The software isn't perfect in its current incarnation. You have to use the algorithm strategically, since it exaggerates all movement. It's also dependent on a training database to understand what it's seeing, so it might take a while before you can completely trust the results. Provided the technology holds up, though, it could be extremely useful for lie detectors and any other device where you'd want to pick up subtle hints of psychological distress.