You can check a reservation balance due (amount due to pay by guest) in the Reservation Form directly. Open the reservation and check the Total Due in the basic information section.
The balance due will be automatically applied when you Take Payment:
You can see that a reservation has a balance due on Bed View, when you see the money bag icon, for example:
And you can preview the balance in the reservation tooltip, when you move the mouse over the reservation, like here: