When editing text, misspelled words or words not found in the dictionary are indicated with an underline.
As you work with text notes, misspelled words are underlined for your review. Right-click to replace the word with provided suggestions or add to the dictionary. The underline is removed from the word when you exit the editing dialog. The in-context spell check feature can be enabled or disabled from the Options dialog.
For more information, see Check Spelling in Text Notes.