Considering how the Apple Watch works as an extension to your iPhone, it would've seemed like a no-brainer to include the ability to view your iPhone's battery life from the Watch, but in true Apple fashion, they did not.
Thankfully, developer Kelly Pangburn did Apple's job for them, creating a simple application that allows you to view your iPhone's battery percentage quickly and easily from your Apple Watch.
Although Battery Watcher isn't the only app that gives us this ability, it is the only that I've found to be free, simple, and not bloated with additional and unnecessary features.
In the Apple Watch app on your iPhone, scroll down to open the options for BW. You'll need to toggle on "Show App on Apple Watch," and I'd suggest also toggling on "Show in Glances" for quicker access to your battery level.
Launch the Battery Watcher app on your iPhone and open its Settings. There isn't much you can adjust (or need to), but you can enable other useful battery information, like standby, Internet, and talk time.
Now on the Watch, you can launch the BW app to view the battery percentage and details, or just use "Glances" to seamlessly lift up your wrist and, well, glance at it. Just note that the Glances view will not include the additional information you may have toggled on.
Apple may include this feature someday, but until then, enjoy Battery Watcher and its simplicity.