you can get a pretty good impression of the face's square by using marker or, ideally, partially dried marking blue and the actual cups, by coloring the BB shell faces and quickly hand-tightening the cups into the frame.

If the cup only rubs off a small part of the dye when you screw it in, there's an obvious high spot and potential problem. Whereas a properly faced, square BB shell will contact the cup quite evenly all around and the dye/marker will rub off the frame and transfer onto the cup along a large part of the circumference.