Can React Native apps be tested in a browser?

Yes! That's very much possible with react-native-web. Here are a couple of related, useful resources to get started:

  • React Native Web Starter
  • A guide to combine React and ReactNative into one codebase

No, React Native can be tested only in mobile simulators like IOS and Android

Update: Facebook has given a beautiful starter pack for React Native.

Introducing Create React Native App

Using this, you can run the app in your mobile using expo app(https://expo.io). It syncs using QR code.