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'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.