Unless you specify it not to do so, Word will check all the words you type and highlights the spelling errors with a wavy red line and grammar errors by a blue or green line. It should be noted that this feature is related to the default language settings used under Word ..(the default language is displayed the status bar of Word).
Errors are not underlined
In the case that Word is not detecting errors:
The option has been disabled.
Word is not detecting properly the language being used.
Click the language displayed in the status bar to open the Language dialog box.