No matter how you charge your Apple Watch—either via its magnetic charging cable or charging dock—doing it in Nightstand mode displays a circular indicator on the screen along with a green lightning bolt inside. The outline gets gradually tinted green to visually denote charge progress, but it's a rough estimate of how full the battery is and as such not very accurate. Here's how you can switch to a more precise percentage display.
How to see battery percentage when charging Apple Watch in Nightstand mode
