We present LibRadar, a tool that is able to detect third-party libraries used in an Android app accurately and instantly. As third-party libraries are widely used in Android apps, program analysis on Android apps typically needs to detect or remove third-party libraries first in order to function correctly or provide accurate results. However, most previous studies employ a whitelist of package names of known libraries, which is incomplete and unable to deal with obfuscation. In contrast, LibRadar detects libraries based on stable API features that are obfuscation resilient in most cases. After analyzing one million free Android apps from Google Play, we have identified possible libraries and collected their unique features. Based on these features, LibRadar can detect third-party libraries in a given Android app within seconds, as it only requires simple static analysis and fast comparison. LibRadar is available for public use at http://radar.pkuos.org. The demo video is available at: https://youtu.be/GoMYjYxsZnI