credit card payments and your credit score

I use my credit cards for almost all purchases and monthly expenses (if no convenience fee is charged) to accrue rewards points but I pay the balance due in full every month by the due date. How do the credit agencies know I pay my balance in full if they take snapshots and there is always a balance (because the due date overlaps with the next cycle)? Do you need to pay the current balance (and not just the statement balance) to ensure a higher credit score? Also I make sure I stay way under my credit card limits.