I'm still getting to know iOS 7, so I by no means know all the hidden gems contained therein, however, when someone mentioned something about a bubble level, sans app, I had to hunt it down. (I already carry a 10ft tape measure in my handbag at all times, so the idea of having a bubble level on me at all times makes perfect sense.) Finding the level is easy, as it turns out. Tap Utilities, then tap Compass. At the bottom of the screen, you'll see two white dots indicating a second page. Swipe the screen right to left to turn the page to the level.
This is what the level function looks like when the phone is laying back-side- down (apparently my office desk is 1 degree off level):