If enabled in the Group Billing Admin > Settings & Reports, when taking attendance you can view who has an overdue balance via the practice/class schedule.
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 on the icon to see how old the balance is, and tap OK to close.