Can iPhone 6 tell if a surface is horizontal?
Yes you can. 


Find a surface that you trust to to be level
Open the compass/spirit level app and place phone on it
Tap the screen while phone is placed on the surface - this will recalibrate the accelerometer reading on your iphone6 implicitly
taking into account the camera bump as well.
Go measure the things you want to measure :)


There is a catch to this, as it does assume that the surface is wide enough to place an iPhone 6 down with the camera bump touching the surface.