If the fields are formfields as used in a protected document, you can run a
macro to check the .Result property of the formfield.

See:

Please Fill Out This Form
Part 1: Create professional looking forms in Word
http://www.computorcompanion.com/LPMArticle.asp?ID=22

Part 2: Adding Automation to your Word forms.
http://www.computorcompanion.com/LPMArticle.asp?ID=46

Part 3: Learn more VBA (macros) to automate your forms.
http://www.computorcompanion.com/LPMArticle.asp?ID=119

Part 4: Use custom dialog boxes in your Word forms
http://www.computorcompanion.com/LPMArticle.asp?ID=127

Part 5: Connect your AutoForm to a database to save input time and keep
better records!
http://www.computorcompanion.com/LPMArticle.asp?ID=136

If they are textboxes on a userform, use the exit event of the textbox to
check the .Text property

See http://word.mvps.org/FAQs/TblsFldsFms/ValidateFFields.htm.

What you are talking about is what Word calls an "online form." For more
about online forms, follow the links at
http://addbalance.com/word/wordwebresources.htm#Forms or
http://word.mvps.org/FAQs/Customization/FillinTheBlanks.htm especially Dian
Chapman's series of articles.

Hope this helps,