Within the app:
1) Tap on the Settings icon in the app
2) Scroll down and click on Version
3) This will provide 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 Google Play 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.