Within the app:

1) Tap on the Settings icon in the app

2) Scroll down and you will see a number next to Version

3) This will be your app's version number

This information can be very beneficial, and often absolutely necessary when troubleshooting issues.

To ensure you are on the latest version, you can always go into the App Store and search for the app.  If the option to Update is in view you will want to update to the latest version, if the option to Open is in view then you are on the latest version.