Sucks don't it!!
I haven't found an easy way.(other than checking it in Solidworks)
If the surface is aligned to an axis then you can project a line down onto it and verify the angle to horizontal.
But if the surface alignment can't be assumed then it's a tedious affair to define a line normal to surface, project it to a horizontal surface, then verify angle between the 2.
If there is indeed an easier way, please advise.