If enabled in the Group Billing Admin > Settings & Reports, you can view who has an overdue balance via the practice/class schedule when taking attendance.
In the upper left corner, tap the hamburger icon to open the navigation panel.
Tap Attendance.
Tap the practice/class.
Tap Take Attendance Now.
An icon will appear, alerting you that the member has an overdue balance.
Tap the icon to see how old the balance is, and tap OK to close.