Before logging in, check if you're logging into Staging or not. In most cases, you will be logging into https://admin.staxbill.com. In some cases, this will instead be https://stg-admin.staxbill.com which will be marked as the staging environment.
After logging in, check the top left corner of the page.
In most cases, the circle in the top left will be blue, indicating this is a standard account.
If it is orange, then this is a Sandbox account.
Expanding the information in the upper left corner of the screen will reveal the company name and account ID.