You can easily verify what exact contacts were counted as "active" in any period for Spur.
Go to Database
Click on "All Contacts" dropdown & change that to "Active Contacts"
Set the billing period to whatever you want
We bill in cycles of 30 days, so if your date is 20th July, the previous bill shall be June 20th to July 20th, and the next will be July 20th
Now just you'll see a list of contacts, all these were considered active in this billing period
To see which message made it count as active, click on that Chat Icon next to the contact and their chat window shall open, then use it to verify if a message was sent via Spur in that billing period.
Change the date range to your billing period
Check for active contacts in that range
Click on the chat icon to go to that chat