You should be notified by a push notification when Monimoto battery level falls below 20%. 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 is shown.
You can also check the red LED on the device. Remove batteries from the slot and wait for 2 minutes, then insert back. If it is blinking one long flash every second or you see no LED - the battery is very low.