You can visit the Apple/Google store, search for the app's name, and review the Version History or App Info.

Alternatively, you can check the app version directly in your device's Settings.

*For iOS devices:
-> Open the device's Settings -> Select General -> iPhone Storage
-> Wait a moment, and the screen will populate with storage stats for each of your installed apps
-> Tap the app you want to see the version number for
-> Next to the app icon you'll see the app name, the version number you're looking for

*For Android devices:
-> Open the device's Settings -> Select App management
-> App list 
-> Tap the app you want to see the version number for
-> Next to the app icon you'll see the app name, the version number you're looking for

Kindly refer to the attached pictures as examples.