boolean devSupportIsEnabled;
ReactActivity reactActivity = (ReactActivity) getModule().getActivity();
if (reactActivity != null) {
    devSupportIsEnabled = reactActivity
        .getReactInstanceManager()
        .getDevSupportManager()
        .getDevSupportEnabled();
} else {
    devSupportIsEnabled = false;
}

public class MyPackage extends ReactInstancePackage {

    @Override
    public List<NativeModule> createNativeModules(ReactApplicationContext reactContext, ReactInstanceManager reactInstanceManager) {
        // store the provided instance of ReactInstanceManager
        this.reactInstanceManager = reactInstanceManager;

        // ...
    }
    //...
}

boolean devModeIsEnabled = this.reactInstanceManager
    .getDevSupportManager()
    .getDevSupportEnabled();