Testing of Android apps is particularly challenging due to the fragmentation of the Android ecosystem in terms of both devices and operating system versions. Developers must in fact ensure not only that their apps behave as expected, but also that the apps' behavior is consistent across platforms. To support this task, we propose DiffDroid, a new technique that helps developers automatically find cross-platform inconsistencies (CPIs) in mobile apps. DiffDroid combines input generation and differential testing to compare the behavior of an app on different platforms and identify possible inconsistencies. Given an app, DiffDroid (1) generates test inputs for the app, (2) runs the app with these inputs on a reference device and builds a model of the app behavior, (3) runs the app with the same inputs on a set of other devices, and (4) compares the behavior of the app on these different devices with the model of its behavior on the reference device. We implemented DiFFDRoiD and performed an evaluation of our approach on 5 benchmarks and over 130 platforms. our results show that DiFFDRoiD can identify CPis on real apps efficiently and with a limited number of false positives. DiFFDRoiD and our experimental infrastructure are publicly available.