When the Keezel battery is running low, the LED will blink red periodically - if the Keezel is in use, if the Keezel is OFF, there will be no LED indication.
You can also check the battery level in the Keezel web-app. Here's how:
1. Turn your Keezel ON. At this point, if the Keezel LED blinks red, then you already know it's low.
2. Connect to your Keezel WiFi.
3. Go to keezel/ (or 192.168.11.1) in your browser and there will be a battery indicator in the top-right corner of the page.