Checking surface level
One thing many iPhone users don’t know of is the leveling abilities of their device. That’s right, you can use your iPhone to determine whether a surface is perfectly parallel to the ground or not. To do this, simply open up Measure and then tap Level on the righthand side.
After Level is open, place your phone flat on its side. You can use either portrait or landscape mode while doing this. Slide your phone across the surface then tap the screen to reset the angle. Do this again across another surface, and your iPhone will instantly tell you where any deviations are.