No more faking out your smartphone or bracelet activity tracker. Scientists have designed a way to train activity trackers to spot the difference between fake and real activity. The new method detects, for example, when a cheater shakes the phone while lounging on the couch, so the tracker will think he’s on a brisk walk. Health care providers and insurance companies are increasingly relying on smartphone and wearable activity trackers to reward active individuals for healthy behavior or to monitor patients.