BCGControlBar Pro for MFC
void CBCGPEditCtrl::EnableErrorIndication ( BOOL  bEnable = TRUE)

Enables or disables edit control error indication. By default, the error indication is disabled.

bEnableTRUE to enable, FALSE to disable.

Call this method to enable or disabled an error indication. If a specific word is "incorrect", a red curvy line will be drawn under this word.

To declare a specific word as "incorrect", you've to override IsCorrectWord() method.