If you're using a battery grip on the Nikon D810 or D800, like the Nikon's MB-D12 or a cheaper alternative, you will have noticed that the standard battery charge indicator on the top screen or through the viewfinder only shows the charge of one battery source--either the internal battery or the grip battery.
But there's a simple way you can see the charge status of both batteries.
First, make sure that the back screen isn't being used. So turn off live view, playback, or the menu.
Then press the "i" button to the right of the main screen. A bunch of settings will appear on the screen. And at the far right is the charge status for each of the batteries.
There are also a couple of other settings on the camera where you can control how it interacts with the battery grip. I have more details in my review of the Vello BG-N7 battery grip.