Where to find the balance
From your dashboard, click the Contracts tab: each contract's balance is shown there.
To see the details, click the contract, then the balance amount.
The three cases
Payments received > amounts due → the balance is positive: the tenant has paid in advance.
This happens when the tenant pays at the end of the month for the following month, or when the matching amount due hasn't been recorded. In that case, record it: see How to reconcile payments and amounts due.Payments received < amounts due → the balance is negative: the tenant is behind on payment.
If that isn't actually the case, a payment is missing to balance it out. In that case, add it: see How to reconcile payments and amounts due.Payments received = amounts due → the balance is at zero: everything is in order, the tenant owes you nothing and you owe nothing either.