Pull down the notification shade, press and hold the settings gear (I forget how long you have to hold, at least a few seconds). A wrench icon should appear next to the gear.
Tap the settings gear and scroll all the way down. Underneath developer options and above about phone, tap System UI Tuner.
Tap Status Bar.
Scroll down and tap Battery. A menu will pop up: Always show percentage, Show percentage when charging (default), and Don't show this icon. Tap Always show percentage.
There will now be a very tiny number INSIDE your battery icon. That number is your battery percentage.