Mini Tank:
The blue light behind the button will not be lit anymore once it is fully charged. The device also shuts off once fully charged. Click the button 5 times rapidly to turn it back on.

When the device is on click the power button once. The power level will be displayed on the digital indicator. If all bars are lit up then the device is fully charged.

Nitro2 and Cyclone:
The USB ego charger light will change to green once the pen is fully charged.  It will display a red light if not fully charged.