At least for some form of mechanical clocks: There is an app for that :)
E.g., for Android, the "Watch Accuracy Meter", which can be found in the Play Store or the APK source of your choice, uses the phone's microphone to measure the frequency of mechanical watches.
E.g., for Android, the "Watch Accuracy Meter", which can be found in the Play Store or the APK source of your choice, uses the phone's microphone to measure the frequency of mechanical watches.