The problem is that with the latest technology batteries, there is no way for you or a battery checker to make a connection with the battery itself...only with the output terminals of the control circuit that manages that battery and protects it (and you) from damage and injury. If you don't have the "secret code" that controls that circuitry, about all you can do is measure the output voltage that it is providing.

You may be able to see minor changes in output voltage to determine whether the battery is charged or not, but probing around the output terminals of a battery can be dangerous, even with their protective circuits. Accidentally connecting a multimeter that is set to measure current could be dangerous to the meter, the battery, and to you. There is an incredible amount of energy stored in a lithium ion battery, and the heat generated by a short circuit can release a lot of it in a very short time.

As has been stated, the battery meter in the camera is your best bet. Even the techniques described in the article using the charger indicator can give misleading resultd, especially with third party batteries that may not provide proper charge indication.