If the BB shell's rim has concentric tool marks made by the circular motion of a facing tool, then it's been faced and you don't need to do it again.

When I see a BB that I'm not sure needs facing, I paint it with a magic marker, or toolmakers dye, then do a light pass with a facing tool. If it removes the dye over 270 degrees, it's fine and I don't have to face. Otherwise I face until I've achieved at least 300 degrees of face, which is needed for supporting cups flat. (low spots don't count, the cup is oriented by the high points of contact, so theoretically 3 areas forming a triangle would be sufficient)

For a home mechanic, you can test by putting a thin grease film on the face or cup, and watching to see if it squeezes out evenly most of the way around. But if in doubt, a skim pass with a facer is to only definitive way to know.