You should be notified by a push notification when Monimoto battery level falls below 20%. Another notification will arrive when battery level hits the 10% threshold. Do not wait until they are completely empty and replace when the first or second notification is received.
Easiest way to get most recent battery level is to connect to device via app in Settings tab. Once you connect - battery is checked and the latest level in HOME is shown.
You can also check the red LED on the device. If it is blinking one long flash every second - the battery is very low.