If you are using the standard App Store version of Abacus POS, you will see the standard Abacus POS icon of the white A in a blue box.
If you are using a Pre-release version of Abacus POS (provided by the team to download via a link), you will see a red flag at the corner of the icon with little "P".
Note: the flag appears for Abacus POS version 2.8.10 onwards only.
You can see the version number of the app that you are using in two places:
On the login screen in the bottom right corner
At the top of the Settings page