To check how much battery life your Apple Watch has left, you have to swipe up on the Watch face to access the Battery glance feature, but you can also add a battery indicator to several of the Watch faces. Here's how to add a battery indicator to the Modular, Color, Utility, Simple, Chronograph, and Mickey Mouse faces:
Just firmly press down on the Watch face until you see the Customize option. Swipe left until you see the individual feature locations and tap a location. Turn the the Digital Crown to choose "Battery" then press the Digital Crown to finalize the process.