To see what your battery usage is like on your Apple Watch and iPhone, launch the Apple Watch iPhone app and go to General > Usage.
You’ll see how many hours of usage your Watch has been subject to since you last charged it. If you think that number looks higher than it should be, chances are something is drawing from the battery while you aren’t actually using Watch.
To see how much battery has been used up on Watch itself, open Glances by swiping up on the clock face, and look for Power Reserve. Here you can see a percentage of battery life left.
Curious about your iPhone’s power usage? On your iPhone go to Settings > General > Usage > Battery Usage and see how power hungry the Apple Watch iPhone app is.