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Easy way to check is to take a panorama photo. If the level is off (as it is in mine) and you follow the on screen level your pic will be skewered.

I'm having to take panos by not using the on screen level but instead holding the phone as level as I can by eyeballing it.
 
I have a iphone 5 and 5S the 5 is perfect 0 with green screen on flat surface, 5S right next to it is showing -3.., there is clearly something wrong with the hardware on the 5S both are on ios7
 
Shows -1 on a flat surface, didn't test different surfaces though. Too bad I'm sending it back becaus of defect Pixel.
 
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