Method 1 - any user

Every user can check the current app version using the 'Shift + i' shortcut when in the app. 

Method 2 - any user

Click the "Help" button (question mark icon at top right) and select "About App" from the pop-up menu:

Method 3 - Jira Administrator

A Jira Administrator can see the app information by going to the 'Manage apps' section and checking the details:

Expand to see the app details: