elksnout, for crowns you are looking for dings, rough edges etc. A Q tip works wonders for finding these. Just run it around the muzzle, on the edge, and if it pulls cotton from the Q tip, there is a burr. Just make sure it is not getting the rifling.
Touch up bluing will just look different, than the original blue job. Slightly off color, patchy. It will just look different, simply because the two kinds of bluing do not blend together well.