This is a prototype of an 'add courtesy accidentals' plugin. It's called 'mark' because a plugin currently cannot add an accidental to a note. Instead, it colors notes red, when there should be a courtesy accidental. It's up to you to add them and to use lasconic's blacknotes plugin to remove the color.
It only works on the selection! If nothing is selected, it'll also do nothing...
Warning: This is ALPHA software. Save your score before trying this!
Due to some other restrictions in the plugin framework, already discussed in http://musescore.org/en/node/17747 it does not work with scores containing:
key signature changes
instruments without a key signature
Accidentals are considered 'missing' when a note in the same octave appeared in a different pitch class in the previous measure in the same instrument (next measure, same octave).