Well, it's likely you don't have much to do. I suspect the head is already perpendicular to the base, so leveling the base should do it.
I'd use a spirit level and shim as needed. Actually, if it wasn't far off, I'd probably do nothing.
Thinking about it, if a fellow wanted to check if the head is plumb, I suppose he could put some sort of grid in the negative carrier and see how it lines up with the edges of a paper easel.
-Pete