iOS 7 has a handy little bubble level that many may not even know about. It’s hidden within the compass.
To access the level, just launch the compass app. You iPhone will likely ask you to calibrate by tilting it in various directions. After it’s calibrated, the compass will pop up.
Now, all you have to do is swipe left (thumb moves right to left). This will turn the page and open the level. Now, lie your iPhone flat on any surface and check how level it is.
The level is two circles that must align perfectly in order to reach 0º. That’s when the surface is level.
Sweet. No need to download an app.