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 app details: