For both the Keypad and the Lock, you can check the battery level from the Settings > Device Info page. Once the battery level falls below 20% though, you'll see a Low Battery warning on the device's Product Page. In addition to this warning, you'll receive a push notification stating that the battery level is low.
When the Wyze Lock's battery gets low, its light will flash red whenever it's locked or unlocked.
For the Keypad specifically, entering your code will still work like normal, but after entering your code the Keypad will flash green, then red to indicate a low battery.