How do I orient a face so it lays flat? I have a simple flat face. I am looking at the top view and the face is at an arbitrary angle. 
The rotation angles show:
x=0
y=0
z=0

I am used to other programs that would shows the relative angle at all times.
How do I turn this plane so it is aligned with the x or y axis?
The transform displayed is the transform of the Object. This particular face has been rotated with respect to object it belongs to, in Edit Mode.

One way to  align the object to its  face, and center it:


In Edit mode, select the face, and in the Header >Transform Orientations menu, hit the little '+' to create a Custom Orientation from the face.
ShiftS > Cursor to Selected
Object Mode, Header > Options menu, check 'Origins'





ShiftS > Selected to Cursor
Header > Object > Transform > Align to Tansform Orientation.


(Uncheck the Origins option)

Now if you set the rotation to 0, the object will align to the world, and its face will follow.