You can tell which version of GP-Billing you are using from the login screen. If you are unsure how to access this, please read:
Logging off GP-Billing. The current version of GP-Billing is 7.28.5
At the bottom of the screen, the shell version of the GP-Billing application can be found (looks like v7.28.5). You shouldn't need to know this number unless support asks for it.
Don't worry if your version number or shell number is different from the versions shown in the screenshots.