How do I know if my Cycloop's battery is empty?

You should be notified by a push notification when Cycloop battery level falls below 20%.  Do not wait until they are completely empty and charge it when the first or second notification is received.

In general, Cycloop's battery can last up to a year on a single charge if the side LED lights are not on.  If the side LED lights are on with an average 2 hour daily ride then the battery will last up to 3 months on a single charge.
If your Cycloop receives a lot of alarms and sends many locations then your battery life might be 3-4 months shorter.