You should be notified by a push notification when Monimoto battery level falls below 20%. Do not wait until they are completely empty and recharge or replace when the first or second notification is received.
(MM1/2/5/6/7 ONLY) 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 does not blink at all - the battery is very low.