You can turn on and off spell checking by entering
^QL. The background of unknown words is getting red. There's no spell checking
dialog window. You simply press ^QO, and WSedit learns all words from the beginning
of the file up to the line of the editor cursor. Some marking of
words below the cursor will automatically disappear. To go to the next marked (wrong)
word, you can simply type ^L. (If spell checking is disabled, ^L is the command for
"search/replace text again".)
If you find an incorrect word which is not highlighted and in consequence the wrong
word is part of the dictionary, you can correct your
dictionary by moving the mouse over the wrong word and
choosing "remove single word from dictionary" in the popup menu.